Just swapped a C4 for a T5 in my '66 and I'm stumped on how to wire in the backup lights. Wiring is not my strong suit, so if anyone could describe this as you would a child please enlighten me.

 

I've done this swap on two different '65s, neither of which had backup lights. That said, I believe there are two possible connections, a set of wires with a plug on the end and the other being a fitting on the side of the transmission with two threaded studs. Grab an ohmmeter or a test light and some wire and test for continuity with the transmission in reverse. It has to be one of the two. Essentially, there is an on/off switch built into the transmission.