Where is the reverse light switch located on a peterbilt 387?

If you have an Eaton Fuller manual or autoshift, it's on the left hand side on top of the transmission... if you have a manual, you undo the screws at the bottom of the boot, lift up the boot, look down there, and you'll see it... there'll be a weatherpack plug connected to it. Disconnect this plug... one pole should be hot with the key switch on. Run a jumper between the two poles... if the reverse lights come on at this point, the switch is bad. If they don't, you have a wiring issue.If you have the ZF Meritor autoshift or an Allison automatic, the reverse lamps are controlled through the transmission computer.