Re: reverse light wires through fire wall on 66
Well back in 66 I don't believe there was wiring run through the firewall. The operation of the reverse lights should have been done through a mechanical linkage called a back drive unit. To operate the reverse lights now you would either need a backdrive unit or run two wires from the pink and light green found at the base of the steering column and just drill a small hole near the shifter on the trans tunnel and then hook them to a switch which is made to operate off the shifter. Don't know if the switch that is made for a Hurst would work on a Saginaw.
I think the back drive linkage was put into cars more around 1969 when they went to the ignition key cylinder on the column and locking steering wheel. The older cars had a switch that was mounted on the transmission that activated the reverse lights. 1966-1968 Chevelle Reverse Switch Kit
71 Malibu, Cottonwood Green, 396 BBC, AutoGear M22W 4 speed, 12 bolt 3:55 posi.