FWIW, you can't plug a 4 pin and a 7 pin in at the same time on the back of the truck. When the 7 pin connector is open at the truck, it blocks the 4 pin lid from opening enough to gain access. So today I played around with the wires in the junction box where the 7 pin cable outputs to the lights to the camper. I'm trying to tap into these wires to run a 4 pin connection to the back of the camper so I can have lights for my small fishing boat I plan on towing behind the camper. I had my truck and boat down there to test it and I had a 4ft 4 pin extension. Right now I'm clueless. I popped a lens off the camper tail lights and there's a black wire, white wire and a green wire. Both tail lights have the same wiring. According to the 7 pin diagram where it runs into the junction box, the red wire is left turn, green [there were 2 green wires so I assume those were running lights] is running and brown would be right turn and white being ground. I hooked it up this way and plugged that into the boat 4 pin and the running lights worked but were very dim and when I turned a turn signal on, both rear lights blinked dimly at the same time. So then I hooked green to green, brown to brown and yellow to red with the same result. I have to figure this out so I'm going to take my voltmeter and start to check continuity on the wires when the 7 pin is plugged into the truck and try and figure out which is which. In the 7 pin junction box on the camper there is 4 white wires bundled and wire nutted with one grounded to the camper [thats where I had my 4 pin ground wire plugged in], 1 red wire, 1 brown wire, 2 green wires, 2 black wires, a blue wire [electric brakes I think] and a yellow wire coming from the truck through 7 pin cable tied to nothing. Any clues? Lol.

 

I would forget about colors and just find them with your VOM or a test light/probe. Do one circuit at a time. The white bonded to the frame is your ground, that is consistent with what I would expect.

probably best to start with pulling the fuse for your battery charge wire so that you can only have once circuit showing 12v at a time. Can do it without, but probably easiest to pull the fuse to start.

plug it all in and turn on your left blinker and nothing else. Start probing the wire nuts in your junction box. Stick one probe on ground or into the wire nut that goes to ground and start probing all the others one at a time. The one that shows 12 volts going on/off is your left brake light.

do the same for the right blinker.

then turn on your running lights and do the same. Make sure the ignition is off or that you pull the fuse for the battery charge wire. The one that shows 12v constant is your running lights. Now go turn your running lights off and verify that the circuit goes to zero.

that’s the only way I’ve been able to do it. One at a time and make sure that you only have one circuit active at a time. I never get lucky with the colors being right so I just always do it step by step and circuit by circuit.

 

I have been double towing for years and have no trouble backing both trailers up, I do have over 5 million miles OTR in a semi with over 2 million pulling doubles and triples, do you think that makes a difference? All kidding aside it can be difficult to back doubles, but not impossible.

 

Mike thank you but this is very confusing to me although I am fairly knowledgeable with a volt meter. Here is exactly what I'm doing at this point. My camper is in a storage lot with no battery because I pull it out for the winter. I drive my truck with boat in tow to the storage facility and position the boat near the camper tongue. I back the truck up to the camper close enough to the hitch so I can plug in the 7 pin connection from the camper. After plugging in, all camper lights function normally. Then I plug in my 4 pin extension to the boat trailer 4 pin. The extension has 4 open wires on one end so I can tie them into the appropriate camper wires to get lights to the boat trailer. Question, while the 7 pin is hooked to the truck with camper lights on and no boat trailer connections made, could I just disconnect lets say the green wire in the junction box and if the camper running lights go off, wouldn't that be the running light connection? Could I determine which wires are for the turn signals the same way? Since there are only 3 wires present at the camper tail lights [white, black, green] and assuming white is ground, the running lights would have to black or green correct? The turn signal connection must be downstream. Sorry, I guess I'm more electrically challenged than I thought but I've wired up multiple boat trailers successfully. I did get the new box for the wire hookups once I figure this out, thanks for the link!

 

Right on! But I did have other lines of work also, so I *ONLY* have a couple million miles. And backing doubles always works best if one can see the second trailer so any input can be seen before things get too far out of line.!

 

Success today. Amazing what you can do with a little instruction and a volt meter. Thanks Mike, once I figured out your instructions, it was easy.

 

glad it worked out.

I was trying to find time today to take another stab at explaining it, so I’m glad you got it figured out.

that new box will make it easy to hook everything back up if you decide to install it that way. Much easier to trace circuits when they are all laid out on binding posts like that. Glad you got it where you need to be.

 

Thanks again! Yes it's a rats nest in there now so hopefully I'll be installing the box tomorrow after a quick trip to the hardware store.

 

pics or it didn’t happen.