82 F800 light wiring dia.

I cannot imagine that is too tough, you are talking Ford right? What can you access? The tail connector? Is the harness "kinda there" but ragged and cut off?

There has to be an access point somewhere I'm not familiar with them. And at that point there can only be so many wires depending on what it came with.........

Maybe some sort of brake warning depending on brakes
Maybe two speed axle wireing
Saddle tanks? Then likely no fuel senders in the rear harness but they may split from that
"Normal" tail/ turn /stop etc wiring, maybe trailer brakes, is it hydraulic or air?