turn signal failure

Can anyone shed some insight on the possible problem(s) of my turn signals not working on a 2001 Impala. The problem occurs after a rain or real humid
conditions. They will work again once the rain stops and completely dries. It could take a few days. Both turn signals are inoperable. Any help would be great. Thanks,
Caddman Canton, MI.
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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. Also check 4 way flashers, if they work ok, and lights are bright, problem could be under dash, if windshield leaks.
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