Crawl underneath with a test light. On the driver's side of the tranny near the rear is a switch that screws into the side of the tranny - it'll have two wires going to it. It's the reverse switch. Pull the wires off. Turn the ignition switch on. Ground your test light - one of the wires should be hot. Jumper the hot wire to the other wire - when you do, your back up lights should come on. If they don't either both bulbs are out, or you have a discontinuity in the wires going to the lights. If they do come on, then the switch is probably bad. If neither wire is hot, then the fuse is probably blown.