1985 Chevy Camaro REAR DEFOGGER

Thats a weird 1, never heard of that before. I would start by checking all of the wiring that goes to the back, you mite have a wire crossed or you mite have a wire touching another wire or they could have melted together. Most likely all of the wires for the rear lights and the defroster are in the same loom, Be patient, troubleshooting wiring is not a fast process. Good luck