Got my 85 back a few weeks ago, new "to me" 351 modified that smokes a bit on start up, needs tie rod ends and all sorts of crap, But one thing at a time. when I picked it up from the guy who put the motor in he informed me that there was some cut wiring near the front tank, the truck had no lights in the rear at all after connecting a few I got it to where it has rear signal lights but still no brake lights or tail lights, additionally it has a dash out of a different truck and apparently that other truck didnt have dual tanks because there is no switch next to the heater/AC control I found the tank selector solenoid with a lonely red wire attached to it but nothing else, There was another smaller red wire spliced into the big red wire on the harness near the tank that also didnt go to anything, Just wrapped around the frame, I have no gauge lights, fuel gauge, speedometer, tail lights or break lights. I have the haines repair manual with wiring diagrams but honestly I cant make sense of it, Im just not good with wires. Please help!

 

Not to sound like a mean ogre (ha!) or anything, but you are going to have to become lots better with wires & wiring diagrams.

I suggest you purchase an EVTM from eBay or any of the places that sell used automotive literature (google it, there are lots of places online) this will show you how things were delivered from the factory, you will have to figure out how to make your stuff work from that.

There's an online 1986 version on Gary's site if you'd rather go that route, too:

1986 EVTM - ???Gary's Garagemahal

As for the speedometer, that's a mechanical cable from the transmission (or transfer case if 4x4) directly to the speedometer, look around at stuff and see if it's connected (or even installed at this point).