I decided to make a thread on this because when I searched for myself, there wasn't a great complete thread on how to do it, and to benefit others. Mods might make this a sticky?
I bought a Tekonsha P2 brake controller through work. Here it is from their website: http://www.tekonsha.com/content/products.aspx?lvl=3&parentid=1400&catID=1435&part= 90885
It is WAY more than I need, but I got a good deal on it. It can control up to 4 trailer axles, and can control electric over hydralic trailer brakes. It also has a "boost" feature which can up the output if you are towing a heavier than usual load.
Doing this yourself is easy, and I think most anyone can do it. The only hard part is working under the dash, and getting up into some tight spaces. Save your moolah and do it yourself!
Hokay. First off, this is what comes in the box:
You get the trailer brake controller (I will call that the TBC from here on), the wiring harness, a pocket mount, and bar mount, assorted butt connectors, mounting screws, and instructions.
I bought a few other things for the install. 14ga control wire (a roll of black for power, and a roll of white for ground), a pack of 14ga ring terminals, a 20amp circuit breaker, and a cheap soldering iron since I can't find my Weller.
Here is a good diagram for the connection you are going to make: http://www.wjjeeps.com/electric_brake_inst.jpg
First, start off by disconnecting the negative cable on the battery.
I started the wiring underneath the hood. I couldn't find a suitable place to run the two power and ground wires through the firewall, so I drilled my own hole, and installed a grommet I had left over from the Autometer gauges I put in my CJ. The hole was a 3/8 bit to fit the grommet. Yours may be different depending on which size grommet you use. I ended up drilling from underneath the dash, underneath one of the main harnesses, right in the middle of the hood release and the throttle cable. I then pulled the two wires into the interior underneath the dash.
Under the hood on the firewall
At this point, I connected these two wires to the harness of the TBC. Black to black for power, white to white for ground. I removed about an inch of insulation from each end, slid on a piece of shrink wrap, wrapped the two wires together for a inline splice, and soldered. Slide the shrink wrap over the joint, heat, done.
Next, go under the hood to make the power and ground connections at that end. Now, I didn't fully understand initially, but the instructions tell you to run the power wire directly to the battery, with a 20 or 30 amp circuit breaker just before the battery. I choose a 20 amp due to the fact that the most trailer axles I will ever run is 2. They suggest if you ever run a trailer with 3-4 axles, put in a 30 amp. Same with the gauge of wiring, they suggest 14ga for 1-2 axles, 12ga for 3-4.
Now for the white ground wire, they emphasize to run that directly to the negative post on the battery. The reason being is that running to a chassis ground can cause noise interference, which can lead to interrupted braking to the trailer, which is bad. Especially in a panic braking situation with your crawler on the trailer behind you pushing you into whatever. I opted to heed their words, and it's easy.
I ran the ground wire along the back of the firewall, following some brake and vacuum hardlines, down the passenger side of the inner fender line, under the coolent reservoir to the battery. I soldered a ring connector to the wire (cut the plastic bit off with a razor blade so you can solder), heat shrinked, and put the ring around the clamp bolt. For the positive, same thing but, you have to add the circuit breaker. The circuit breaker protects against a short in the system or a overload in power. It is self resetting so you shouldn't have to worry about it. The breaker posts are labeled "to aux" and "to battery", so mount accordingly. I found a good spot above the coolent reservoir on the pinch weld of the fender. There was already a hole from the factory there, so I drilled out the hole bigger with a 1/4 bit I believe, and used a bolt and nut to mount.
Cut the black power wire where it will end at the "aux" terminal of the circuit breaker. Strip, crimp, solder, heat shrink, mount. Now make a length of black wire from the "batt" terminal of the circuit breaker to the positive clamp of the battery. Strip, crimp, solder, heat shrink a ring terminal on both ends of the wire. Mount to breaker, and mount to tightening bolt on battery clamp.
Tighten up the wires along the run from the battery to under the dash, and zip tie them down along the run so they are nice and neat. I think I am going to go back when I have time and put these wires in a split loom so you can't see them, and zip tie the loom.
Now to the fun part! Under the dash!
There are two more wires to hook up under here. The blue wire, which controls the actual voltage going out to the trailer through the 7 pin connector to power the trailer brakes. And the red wire, which connects to the brake lamp switch output for the stoplight.
The blue wire is up behind the fuse panel in the upper left corner under the dash. There are a lot of wiring looms in this spot, and I had to hunt for it, but it IS there, IF your WJ came with the factory tow package. Here it is pulled out:
Inline splice this to the blue wire on the TBC harness. Strip, twist, solder, heatshrink. I orginally used a butt connector for this, but I ended up soldering every other connection, so I cut the butt connector off and soldered this one as well.
Now to the really fun part! You are looking for the white/tan wire connected to pin 5 of the brake switch connector. Open up the loom above the connector to find the wire. This is it here:
Originally I had been told to use a scotchlock splice here, but when I did, it didnt cut into the tan/white wire at all due to the different size of it and the red wire from the TBC harness. This caused the TBC to not function correctly under regular braking with the trailer hooked up (no trailer brakes!) I decided to solder it.
Very carefully use a wire stripper to cut about a 1" section of insulation out. I ended up kinda cutting the two sides with a stripper, then using a razor blade to cut along between the two, and then peeled the insulation off. Be very carefull here as you do not want to cut any of the copper wire. I then cut 1" of insulation off the red wired from the TBC harness, separated and made a "gap" between the copper wire on the white/tan wire, and "braided" the red wire into the white/tan wire. Sure, this is overkill, but I wanted to make sure the wire had the best possible contact. I then soldered this connection, and used electrical tape to wrap it up and protect it. Unfortunately, I did not get a picture of this connection.
I should also add that when you are wiring under the dash, make sure you don't get wire tangled up, so you can coil up the excess, and zip tie them up nice and neat!
With the wiring done, time to mount the TBC. I was going to mount it by my left leg under the edge of the dash, but figuring I am a righty, I decided to mount it to the right almost against the center console. I used the pocket mount as it hold it up tighter to the dash, and I wouldnt be able to put the screw in the right side of the mount if I used the bar mount.
Line up the pocket straight under the dash, and drive in the two self tapping screws to mount it, making sure you do not screw through anything important on the other side up underneath the dash. Slide in the TBC till it locks in place, plug in the TBC harness connector, and reconnect the negative battery cable.