Originally Posted by LATECH
bad ground at the front right hand lighting harness.Or the ground wire itself.
Trace the harness back from the bulb in question.You will see where the black fastens to ground a foot or two from the bulb.
^^^THIS. There is a bad ground somewhere. The 1157 bulbs in the turn/stop/tail lights have two filaments that share a common ground point. If the ground is bad, the turn signal circuit will ground through the tail/park circuit. This works so long as the lights are off, but when you turn on the lights, that back-feeds 12V to the tail/park circuit, which prevents the turn signals from working. The easy way to find which one is the problem is to look at the bulbs when the turn signals are operating and the tail/stop lights are off. The bulb that has both filaments illuminated is the one with the bad ground.