Help ID'ing wires

[Buckethead03]

I have a 71 that i am swapping cabs on. The original cab is from a manual everything truck with no a/c with a stock 302. The donor cab is from a 71 with power everything and it does have a/c. It was powered by a 390. I am in the process of wiring everything back up but have run into an issue with one of the stock harness plugs. On the passenger side firewall is a main harness containing the headlight, solenoid, and horn relay wires. I have everything hooked up that i can see on the engine side but i have a flat 3-wire pigtail that has nothing to connect to. It has what looks to be a 10ga or so black wire with green tracer, a yellow wire, and a red wire. I went back to my original truck to look for the same pigtail but its not there. What i found was an extra set of wires hooked to the starter side of the solenoid. This factory ring terminaled pair contains 2 of the three wires from the mystery pigtail, the black/green and the yellow. The red wire from the mystery harness is nowhere to be seen on the original truck. Does anyone know what this wire might be used for? I touched the black/green and yellow to 12 volts like it is on the old cab and everything works in the truck. Nothing worked before. I just can't figure out what the extra wire is for.

next question...

I have an orange wire coming out of the drivers side firewall area that also has no connection on this truck. It looks like from a schematic posted here that this wire is for the fuel tank sending unit, but i can't find anywhere to connect it. where should this wire go in the engine compartment? the only other plug that looks like it could connect to this wire goes to the carb solenoid.

Any ideas/solutions would be greatly appreciated

[Swegner99]

That orange wire (if its for your sending unit) should connect to the back of the fuel gauge, If I remember right...

On a side note, I didn't know any of these trucks came with factory power options... Maybe I just hadn't heard about it, are you referring to power windows and door locks?

[Buckethead03]

That orange wire (if its for your sending unit) should connect to the back of the fuel gauge, If I remember right...

On a side note, I didn't know any of these trucks came with factory power options... Maybe I just hadn't heard about it, are you referring to power windows and door locks?

Sorry. I meant power steering and power brakes only. That was power everything back then.

The orange wire is only about 4 inches long and there is no other wire to attach it to.
There isn't another wire coming from inside the cab through the firewall that I can see. The wire is connected to the sending unit in the tank.

[BobbyFord]

could the short orange wire be for an auxilary fuel tank?

[Buckethead03]

could the short orange wire be for an auxilary fuel tank?

I was wondering the same thing. It doesn't look like its ever been used to be honest and I don't remember unhooking it from either truck.

[71swb302]

I have that same Orange wire on my 72. Couldn't find anywhere to hook it up either. Was wondering the same thing....

[Buckethead03]

I did some more digging on the original cab and found out that the Black with yellow, not green tracer is a hot wire for the electrical stuff in the truck. I also figured out that the smaller yellow wire is looped around and taped up and hooks into the horn relay pigtail. I am still unsure what the other red wire in the 3 pin pigtail is for. Maybe something to do with the 390?

[Rattler12]

The orange wire could go to the dome light in the cab.