As Clinto describes there are several ways to do things, the best is in such a way that the trailer brakes are actuated by your own brakes either by air pressure OR by the brake light switch on your truck. Depending on the size of the trailer you're towing, you could get into REAL trouble if the brakes on the trailer aren't working when you need them.
The best way is a 24-12 volt converter in the cab. This allows you to monitor the brakes on the trailer for function AND allows you to manually actuate them (also handy for backing up and setting them to bump the automatic adjusters in a purposeful manner
). The 5 tons have an extra lead on the 12 pin connector that provides 24 volt power so if you want to run a battery charger back there for the breakaway circuit you'll have to do something else on a deuce. I wired up that line on my deuce for 12 volt power to the rear (on the same pin) AND ran an additional pair of wires to the back of the truck from the brake controller. An electric brake controller MUST have the ground and 12 volt brake signal on it's own paths and they MUST be large enough for the current load. Having a fuse pop, a breaker blow or worse, a wire melt because of the current draw (too many amps) during a panic stop is NOT the idea of a good day to me.
So, you've got 12 volts in the cab, you run two wires from the 12 volt supply. one goes to a 20 amp breaker or fuse and thence to the brake controller. One goes to another breaker or fuse and to the trailer power connector, check the wiring, I can't recall the pin, but you can break the connector down and solder a wire to the appropriate pin to get the straight power lead to the trailer (check a van trailer TM for the correct pin, I think it's H now that I'm thinking about it.) But for your purposes just run the lead to the back of the truck. We'll tap that into the trailer a different way.
The power to the brake controller gets spit. you need to TELL the brake controller when you're on the brakes right? That's a switch on the brake pedal BUT that's currently running 24 volts as a signal to the brake lights. You can't run that in to the controller, it'll let the smoke out. So, I opted for a second brake signal light on my truck. Deuces have two places for them on the brake master cylinder. There's an Air line brake signal switch and an hydraulic pressure brake signal switch. Figure out which one you don't have and order that part from Saturn Surplus. If you don't like the idea of this you could go with a relay and work it that way OR add in an additional switch on the air line. There are several ways to do this, non is really the best way.
Mount the brake controller where you can glance at it. I can reach down with my left hand and find it and hit the manual switch as sort of a check to see if the trailer is there or not. It also allows me to, on wet ground, check to see that every tire is locking up when I start a trip. Start a shallow turn slowly advance at 5mph and bump the lever you should be able to see the brakes actuate on each tire and lock that tire. Good way to know that the brakes are working. No working brakes, again, a BAD thing. I just wish I could easily check on the deuce this way.
So, run the 12 brake and ground lines back to the trailer along with your 12 power signal. For your purposes, you're wanting to not modify an existing trailer right? So, the trick here is to find a plastic project or electrical box from home depot's electrical department or Lowes or an electronic supply store. Get yourself a civilian trailer connector that matches the type your target trailer has (make sure brake leads are included). Build yourself a male to male cable with a receptacle for your cable on the adaptor box. you can permanently mount it in the truck under the frame OR you can toss it in the bed. It'll have some mounting feet, so you could put some strong magnets on it so it doesn't slide around in the bed.
So now you need to provide power to your trailer lights right? Well, you can run a trailer connector from the 12 pin on the truck OR tap off of the lights off the rear harness. Add in a splitter, run that to the control box and run those to the appropriate pins on your civilian receptacle.
So, you now have coming into the adaptor box, 12 volt power, brake ground, 12 volt brake signal, Brake, left turn signal, right turn signal. Add a ground lead from this and run it to the truck's frame somewhere. For basic power, you can never have too many ground leads. Connect those leads inside the project box to some screw type terminals (radio shack) and run them through the sides of the box through a sealing bulkhead conduit fixture that you get from Home Depot or some such (you want the box to stay water proof). You can install a conduit fixture with a downward bend and squirt some silicone caulk into it for a simple way to seal it up.
So, now, the question of how to get the brake and trailer lights to work. Do you run a set of relays or do you swap the bulbs. One way is more fiddly, the other is more time consuming when you're borrowing the trailer. You can find most bulbs in 12 volt in a 24 volt form, though it is harder with some types. Some can be found in 12-24 volt form in LED format (a nice upgrade) if not cheap.
Relays are a nice way to go. Bjorn will endorse these as well. they're pretty simple electrically too. Basically it's a coil (wire wrapped around an iron core) with a reed switch. You have 4 key points on a relay to wire. The coil, (two pins) and the switch (two pins). When you have power running through the coil, the switch closes due to the coil turning the iron core in to a magnet and closeing the reed switch. Certain relays are rated for certain voltages and amperages, but in this case it's low so you should be able to find 3 at radio shack for your purposes. Oh, if you have a frys nearby that'll work too, probably better source of eletrical parts as well.
So, wire your trailer lights to the switched parts of the relays. Run the supply for those to the 12 volt power coming into the box. Those relays can be found with sockets so you can mount the sockets in the project box and move them around as you choose for testing or replacement. The coil sides go to the truck's brake, and turn signal lines to each of the specific relays for the trailer.
Connect your now 12 volt signals leads for brake, and turn lights, your brake 12 volt and ground and the 12 volt power lead to the civilian trailer socket in the box and start testing this.
A Digital multi meter is a VERY good thing to have to start trying to understand how basic electrical components work. There's nothing fancy here except for separating voltages and making sure you're not making a short (12 or 24 power direct to ground). For the purposes of operation, a coil while VERY low current draw DOES have a current draw and while taking a lead in putting it to ground is a short, putting the same lead onto a coil for a relay does not create a short. The coil has enough distance that it has resistance and thus some current draw.
Test your work, make sure it does what you want. By using screw terminals strips, you can move wires around if you make a mistake instead of having to cut and resolder wires. Use terminal strips, some good terminal crimpers and you won't have to solder, though a soldered connection is better. You could crimp them all, then go back and solder and seal them. I use the paint on electrical tape for sealing electrics because that gets into cracks that tape and heat shrink doesn't get into.
Mind you, since I wasn't swapping the trailer with someone else, I went ahead and kept the system jut using my own brake and turn lights in 24 volts. That is easier in design BUT as I said, having to swap the bulbs out or replacing all your friend's bulbs after you forgot and blew them on 24 volts is a pain.
A side note, with the deuce's suspension and the tag setup, get the weight as far forwards on the trailer as you can, that'll have the least amount of bounce. The Lunnette eyes can take it unless we're talking about a serious amount of weight. A 3" lunnette eye can handle FAR more down force than a conventional hitch and ball can. Squirt truck can explain this more since he breaks things like this for a living. I also modified his idea for a trailer adaptor to my own purposes.
