I recently picked up a '84 720. I had to put a flat bed on the truck and was going to use trailer lights as the stop, turn signal and tail lights (The backup and license lights will stay on the factory bumper). I've got every thing but the brake lights working correctly. There are four wires on the truck and only three on the light box (ground, tail, stop/turn). If I wire the brake light wire on the truck onto the same wire as the turn signal, the turn signal will stop when I hit the brake pedal. Does anyone know how to fix this? I've reached the extent of my electrical knowledge for trucks!

You may be able to use a taillight converter commonly used to adapt trailer wiring harnesses onto the existing vehicle harness.

Here's what I'm thinking of:
http://www.etrailer.com/Wiring/Draw-Tite/119175KIT.html

After I posted the first post, I did try the converter. However, I wired up the 5 wires from the truck (Turn signals, tail, break and ground) but the voltage still dropped to zero when the brake was pressed and the turn signal was on.

Does anyone have or know of a wiring schematic that's out on the web? I must be missing something obvious but for the life of me I can't see what.

1981 wiring diagram should be fairly close:

Thanks, CaptainZeros! I'll take a closer look.