Car Starts in Gear

An original '66 shifter (either floor or column) would just have a backup light switch. The NSS is a separate part, and just a single wire version in '66. It has to match a single wire rooster comb in the transmission. If you use the 3 wire NSS with the matching rooster comb, you need to attach the factory single switch wire to the center pin on the 3 wire NSS. If the stock wire is connected correctly to the starter relay and center pin, and it still starts in all gear positions, you either have a bad switch, rooster comb, or the wire is shorted to ground somewhere.