Backup lights

You should have wiring if it's the original harness, '66 was the first year backup lights were required on cars. There should be some wiring around the trans since that is were the switches were for the backup lights.

It helps to have a Mustang wiring diagram manual. Back-up lights are wired up through the driver side of the car, through the dash, and either through the fire wall down to the automatic transmission (follows the neutral safety switch wiring) or under the carpet to the transmission/driveline tunnel on standard transmissions. I think is a black and red wire.

Welcome to the forum.