My reverse lights dont work what could be wrong?

If a fuse is blown, the circuit won't work intermittently. "Everything else" working means nothing because there's dozens of fuses for those other circuits.

Suspect the neutral safety switch first. When the lights are not working, unplug the connector from the switch and use a stretched out paper clip or piece of wire to jump the two outer terminals. If the backup lights come on when the ignition switch is in the "run" position, but not when the switch is plugged in, the switch must be replaced.