Where is the backup light switch on a mustang 3.8 V6 automatic?

There isn't a separate "backup light switch" in the way you might be thinking on a Mustang (or most modern cars). The backup lights are controlled by a reverse light switch which is integrated into the transmission. It's not a switch you can flip or replace easily; it's a sensor that activates when the transmission is shifted into reverse. If your backup lights aren't working, the problem is likely with the switch itself (inside the transmission), the wiring to the switch, a blown fuse, or a problem with the lights themselves.