1985 GMC Brigadier

[Jizzo17]

Hello everyone. I recently purchased a Brig to use as a farm truck and am trying to get the tail lights and a 7 pin plug for a gooseneck wired in. Unlike my 85 Mack, this truck does not have a 14 point junction for the wires on the engine side of the firewall. It is set up with a glad hands and the 7 pin connector for a trailer and I could just tie in to that but I was hoping to do it a little bit cleaner than that lol. 
This truck does have the Cummins and 7 speed with a low ratio single screw rear end. 
 

Any help or wiring diagrams would be appreciated. 
 

[Geoff Weeks]

Look along the frame rail, there should be a 4 or 5 wire cable for the lights. Unless someone has totally ripped this truck apart, that is where the factory would have run the cable.

I have diagrams for 1973 but not newer. They might tell you the color codes for the wires as those tend not to change much over the years.

[mowerman]

Great score we would love pictures… bob

[70mackMB]

l had one of them. Cummins - 7spd - single axle - one seat - manual steering - spring ride - Swamp Hollow Orange paint color but it said Yellow on the door to confuse the children. Made more money per mile with that truck than l did with the other ones l owned! lol    .....Hippy 

[Jizzo17]

Found the junction box behind the passenger seat. Tucked in behind a black plastic cover. 

Thanks again for the help and here are some pictures. It was originally orange as well and the odometer shows 250k which I would think is about right for how clean it is. 

[Jizzo17]

Link to the pictures since I couldn’t get them to upload

[Geoff Weeks]

1 hour ago, Jizzo17 said:

Found the junction box behind the passenger seat. Tucked in behind a black plastic cover. 

Thanks again for the help and here are some pictures. It was originally orange as well and the odometer shows 250k which I would think is about right for how clean it is. 

Is that JCT box for the 7 wire trailer or for the truck tail lights? I can see the 7 wire being in the cab but the lights on the rear of the truck generally run down the frame rail. You need wires for reverse lights that will not be on the 7 wire trailer cable, plus they generally only have two lights on the rear of the truck, so need a different feed from the turnsignal switch to incorporate the brake light function as well.

[cruiseliner64]

2 hours ago, Jizzo17 said:

Link to the pictures since I couldn’t get them to upload

Real cool old school truck.I like it...

Paul

[Jizzo17]

@Geoff Weeks, that junction box is where both the tail lights and the 7 pin plug for a trailer both meet. The 4 wire is ran from the rear up to the cab on the driver side then along the under side of the cab to the passenger side where it pops up through what looks like a factory hole in the cab corner floor. The 7 is ran from that box inside the cab to a hole somewhat behind the drivers seat. Where it exits between the glad hands. 
 

@cruiseliner64, thanks. Me and my brother think so too. We are planning to put a rear axle out of a wrecked truck in it so we can convert it to 10 lug and so the same for the front. But that later on down the road. 

[mowerman]

Thanks for the photos. Looks good and really doesn’t look like it’s been beat to hell. Good luck with it.

[Geoff Weeks]

10 hours ago, Jizzo17 said:

@Geoff Weeks, that junction box is where both the tail lights and the 7 pin plug for a trailer both meet. The 4 wire is ran from the rear up to the cab on the driver side then along the under side of the cab to the passenger side where it pops up through what looks like a factory hole in the cab corner floor. The 7 is ran from that box inside the cab to a hole somewhat behind the drivers seat. Where it exits between the glad hands. 
 

@cruiseliner64, thanks. Me and my brother think so too. We are planning to put a rear axle out of a wrecked truck in it so we can convert it to 10 lug and so the same for the front. But that later on down the road. 

Ok, I'm trying to wrap my mind around what you are saying. SO is there separate wires in the box for back-up, stop and turn combo for left rear truck, and stop and turn combo for right rear truck plus tail, and  separate turn and brake for the trailer 7 wire? Seams like too much for a single JCT box. 

Trailer turn (7 wire) are connected to the FRONT turn signals on the truck, while being supplied a brake light feed that is independent of the combo feed from the turn signal/brake combo the truck uses.

That is a lot to stuff in a Jct box.