This is a semi-brief overview of what it takes to put a brake controller into a tow rig. I'll start with a typical hard wire set-up, where you have to run all of your stuff from finding power to wiring the plug. After that, I'll start into factory tow package stuff.
Everything I'm going to outline here is assuming nothing is trick about your tow rig, and nobody has gotten creative with the wiring. If you decide to take on this project and have any questions, feel free to ask. I'll try and answer the best I can.
A hardwire brake controller
First things first, look around the back of your rig and find out if you have wires run from the factory to run lights. If your rig has them, they could be wrapped up around each other and tucked up out of the way. Most times it is easy to spot them. Suburban's have them up under the drivers rear fender, and Ford's and the newer Chevy's have them wrapped up and taped to a wire loom behind the bumper... You get the idea. If you and I can't find them, you could probably stop by a hitch place and have them take a look. Some rigs need to have a T-connector, and others may have a plug that you can pull your wires from, or plug a harness into. I found a really good link while doing a little research on this where you can get most of any tow rig parts. While you are there, pick up the female connection you need to plug your trailer into. This link will also give you some different options as to how to mount the plug.
http://www.hitchcorner.com/index.htm
Go down to your local auto parts place and pick up a couple of self resetting circuit breakers. Get one 20 amp and one 30 amp.
Pick up 25 or 30 feet of blue 12 guage wire, 30 or 35 feet of black 10 gauge wire, a 10 feet of white 12 guage wire. That should be more than enough wire, but it's better to have too much than not enough.
You need a couple of electrical ring terminals too. You need five #10 ring connectors for 10-12 guage wire (the yellow ones). Decide where you are gonna pull your power from. Different trucks have different places to pull power from, and any given truck has several different places to do that. You need to find out where you are going to do that, and get any terminal you will need to fit over the stud that has power. Could be at the battery, at the starter, or from the feed on a buss bar. You also need a couple of blue butt connectors, a couple yellow butt connectors, and maybe a couple of different sized scotch locks. Unless the controller you buy comes with one, you will need at least one blue one. Zip ties, black tape, and split loom also make the job a little cleaner. Go ahead and pick up a small assortment of other electrical terminals just to have them on hand if you find a place where one would work better than another. Pick up a couple of 1/4" self tapping screws, and a box of #10 self tapping screws, to mount stuff and attach grounds. The ones with the hex head work best in a cordless drill.
Find someplace under the hood to mount the circuit breakers. You want them to be as close to power as you can get. Under the hood of these late model trucks can be really crowded, so good luck, you may need to be creative. Use a couple of #10 screws to attach them to something. Watch and make sure that you don't punch through anything important, and that nothing is going to smash the breakers when the hood is shut.
On the circuit breakers, there should be two different color posts. One silver, and one brass. If they are the same color, they should be marked 'Batt' (the brass post) and 'Aux'(the silver post). 'Batt' is the power side, and 'Aux' runs to the controller or the plug. Strip the ends off of a chunk of black wire long enough to go between the brass posts on the breakers. Attach a #10 ring terminal to each end and put them on those posts.
Run the black and blue wires up from the back of the truck. Don't worry about making them permenant yet, but run them how they are going to be strung, i.e.: over, around, and through crossmembers and brackets. Pull those wires all the way up under the hood. Pull the black wire up to the 30 amp breaker and attach it with a ring terminal. Pull the blue wire up and feed it through the firewall. I usually can find a grommet already in the firewall that I can fish the wire through. Sometimes it is neccessary to drill. If you have to drill, look at where you are going to drill through, and what is on the back side of the firewall. It will ruin your day if you drill through the firewall, and tag the fuse box, or if you go the other direction and punch the brake booster. Leave enough slack up under the hood that you can pull more wire through if you need it.
Mount the controller under the dash. I usually mount them down low and to the right. That keeps it down out of the way but still accessible to make adjustments and reach the manual slide lever. Mounting down low also makes it easier to remove the controller without the screw holes showing. When you finally trade your pig in on that new hi-po diesel whatever you have been saving your pennies for, you will get a better trade if your dash isn't all torn up. Other people that tow often like to move the controller up closer to within reach. The manual control lever will save your ass if you ever tow in conditions that are less that ideal. If you save your road trips for more fair wether, you won't have to worry as much.
Pull the blue wire up and connect it to the brake output wire coming from the controller (usually blue). Leave enough slack so that you can route those wires up and out of the way when you are finishing this job up.
Now you can worry about making the wires under the truck permanent. Start at the firewall, and work your way back down the truck. Stick a handful of zip ties in your pocket and go to town. I try and tie the wires right to the wiring that already runs down the frame. That way it stays up out of the way, stays away from the exhaust, and stays away from anything that could rub through the insulation. Pull them all the way back to the bumper, but leave enough wire to work with when you wire the plug (bout 12"). Snip off the leftover from the black wire. While you are doing this step, you can make this as clean as you want. All it is, is time. You could probably fish the wires right into the existing wireloom, or you could run another wire loom right along the factory's, or don't use any at all. It just makes it look a little cleaner, and gives a bit more protection for those wires.
Attach a ring terminal to the black wire you cut off the back, and hook it to the AUX terminal on the 20 amp breaker. String the wire through the firewall where you ran the blue wire. Hook that black wire to the power wire of the controller (usually black). While you are under the dash, attach the white wire you bought to the ground wire on the back of the controller (usually white). String that through the hole in the dash with the other wires. Run that out into the engine bay, and hook it to a good ground somewhere. I usually run to the ground bolt on the core support, unless I can go right to the battery. Battery is the prefered spot to run for ground. Sometimes, if I have to, I run straight down the firewall, and take that ground right to the frame with a 1/4" ring terminal, and a 1/4" screw. It all depends on what is cleanest for me.
The only wire left to hook up on the controller is the stoplight feed wire (usually red). That goes to the cold side of the stoplight switch. There are a number of different places to find that wire, and if I can ever find the book I have at home, I'll post those up. Use any of the splice connectors that you prefer, but a simple scotch lock will work fine.
Tie all of those wires together and tie them up out of the way. Clean up the under side of the dash so that the next time you add something, or if you ever have problems, it will be easier to find the wires you need. A little bit of split loom will go along way here. I've seen some professional installs, that I have to work around, that look like complete trash. I've had to stop and clean up other people's mess more times than I care to count, just to get my stuff installed.
Go to the back of the truck and put in the wiring T-connector. Cut the 4 flat plug off of the end of the T and wire up the 7-way plug at the back. There are a couple of diagrams already here in the towing section, but I will find a good one, or draw one myself, to post here in this thread. There should be the standard color wires on the T. White is ground, brown is running lights, green is right turn, and yellow is left turn. Black is the hot wire, blue is the brake wire, and use the leftovers of the white wire you used for the controller ground for a larger ground on the plug. You may have an extra wire hangin around. You may have a back-up light wire. If you use a 7 way plug, that wire will go to the center pin.
Bring the big white wire out of the plug and attach it to the frame with a 1/4" ring connector and a 1/4" screw.
Now go back up under the hood of the truck and hook up your power. You are probably running out of the black wire, but you only need enough to go from the brass post on the 20 amp breaker to where ever you found best to draw power from. This will put two terminals on the 20 amp brass post, but you want to get power to that post first. That way, the controller is always the first thing to get power. Anything can happen to that wire running between the breakers (gets cut, corrosion, etc...), but the controller will get power... Not having interior lights on your enclosed trailer is way better than not having brakes.
Scott Eickerman