the taillights (brake and turn signal) work fine when the headlights are NOT on, but when you turn the headlights on, the back lights up fine UNTIL you step on the brakes, then the passenger side cuts out all together...any ideas?

 

Check the grounds. Bad grounding can cause some wierd things to happen.

 

The long wires in the sides of the bed likely have bad insulation, carefuly swap those out with better wires & you should be fine, also - the sockets for the bulbs in the back may be corroded, easy fix for little cash if that is the case.

 

Well, ran a ground wire from the good side and problem solved.