So I am building a off road camping trailer and I need to wire it up. I am going with a 7 wire plug, with the eventual plan to be able to charge a battery on the trailer through the alternator. My first question is do I want a junction box on the trailer to wire everything into? Or is it just another point of failure? Secondly, if I do install a junction box, do I want it closer to the tongue (and the cheaper wire from the tail light and brakes up to the junction box) or should I get I get the longer 7 wire plug and install the junction box to rear of the trailer and then just have to run short wired from the lights/etc?


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Don't know if it's right or wrong, but when I rewired mine I didn't use a junction box... just spliced into the pigtails, soldered the connections and covered 'em with heat shrink tubing.  I can see where a junction box would be nice, though, especially if you're planning to tap auxiliary power from it.

I did all my splicing up front, then ran the brake and taillight wires through plastic conduit to the back of the trailer.  Probably overkill, but it was cheap and I figured it'd help keep the wires from possibly sagging and catching on something (I sometimes drag my trailer out into fields to pick up firewood or whatnot).

In the same boat, I don't know what's right or wrong but I ordered a cable and box today for my 5 ton trailer.  I see the trailer plug cable as a replaceable item as I have drug a few down the road.    

Cheaper and/or easier to replace a 6' cable than either the plug itself or a heavy 15' cable.

Use a short 7-pin plug with the junction box in the front near the tongue. Then run your wires to your other items and use protection. You can run them inside the frame (be sure any holes they run through have rubber grommets to prevent abrasion) or along the frame in conduit or sleeves.

If you have electric brakes, make sure the wires from the junction box to the brakes are 12AWG. If you're using LED lights, you can run 18AWG for those. Any splices you make should be done using adhesive-lined heat shrink butt splices like these. Make sure to do a pull test after crimping the splice. Another piece of adhesive-lined heat shrink on top of the splice is always good too.

I design military vehicle wiring for a living. I can make your trailer harness nearly bombproof if you throw enough money at it

I would assume you're gonna mount the battery/box on the tongue not inside where it could out gas. I would mount the junction box near it, maybe just inside the front of the trailer. Forget the crappy yellow, brown, green 18 gauge wiring kits back to the brake lights. Get a 50' contractor grade 3-12 or 3-14 extension cord and run a separate length back to each light. Solder, RTV/silicone and shrink wrap all linemen's splices. Bulletproof.

Run a ground wire for ANY voltage (NOT a 'frame ground').

It matters a LOT more for low voltage.

120 V will make a decent connection though corrosion.

12 or 24 V WILL NOT.

You have to move up to at least 48 V for reliable connections even with the best of techniques.

Do you mean to run a ground wire back to the ground on the fuse block for everything I add?

Yes.
Just by a roll of green wire.

Easy enough. Thanks.

Also! I'm installing led tail/brake/turn lights. I was told I need to install a resister?