Where is the back-up switch in a 1966 ford mustang with automatic transmission?

A 1966 Ford Mustang with an automatic transmission doesn't have a dedicated "back-up switch" in the same way some later vehicles do. The backup lights are activated by a switch within the transmission itself, which is engaged when the transmission is shifted into reverse. There's no separate, accessible switch to troubleshoot. If your backup lights aren't working, the problem lies within the wiring, the transmission's reverse light switch (which is often part of the neutral safety switch), the lights themselves, or their fuses/circuit breakers.