I have tail lights/flasher lights on the outside/bottom of my ROPs, but I can never remember seeing any on the back of the fenders, ever. I'd think that on a tractor, having wires running inside a rear fender would be "courting danger" because of mud and debris picked up by the rear tire cleats. They would probably have to run a conduit for the wiring or it would constantly be ripped out. ...just my opinion.
JimI