Could be bad ground, turn on taillights, are they bright, or dim. If dim,
remove ground screw, clean terminal, replace screw with Stainless steel one,
paint over to keep connection rust free. Two grounds are used, one goes to
rear bumper, other one goes to radiator support. When grounds are known
good, get spraybottle, add some water, and squirt around with signals going.
4 way flashers, if they work ok, and lights are bright, problem could be
under dash, if windshield leaks.