I have a 75 that I am converting from a 1.8 to a freshly built 2056, the back up light plug is connected to the harness off the 1.8, what/ how do I connect it when I put the 2056 in? I am also going with carbs not FI.
HELP!!

I believe the wire for them is grey with a blue stripe. Not sure where it plugs in though.

what? that circuit should be contained in the wire loom that goes to the starter. independent of the fi harness.

yeah, the wire loom connects the following:
starter, back up light, relay board, grounds (on case), a relay on the firewall and the coil.
Anyone know exactly how they go?

If your 1.8 had the stock FI, then that loom is the ignition harness and it goes from the 12 pin connector at the relay board to the coil, oil pressure switch and half the dual relay, and part of it splits off to the starter and backup switch. According to my Haynes manual, those two wires from the backup light switch come from pin #2 and pin #4 of the 12 pin connector on the relay board.

I can't think of how'd you could avoid using that ignition harness. You could probably modify it by removing the dual relay holder and acompanying wires.

do I need to do anything with the other 1/2 of the dual relay and it's accompanying wires?

I think you could just zip tie it together and tuck it out of the way. Depends if you want to preserve it for future use. Some might say to just cut those wires off and cap them.