If they work with the headlights turned off, you've got a bad ground between the bed and the frame. The turn signal is backing up through the tail light wiring to find a ground until voltage goes onto the tail light wires. Since current can't flow both ways through a wire, the turn signals quit working with the tail lights on.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!