Turn signals not flashing

Dana hit it right on the head, vise-grip one end of a length of wire to the neg battery post, touch the other bare end to the actual metal bulb base, lites should work as normal.
With a poor/no ground at the bulb, the power comes to one filament of the bulb, but with no ground, - power goes up thru the other filament, (both filaments lit) back up that wire to ground at the indicator bulb on your dash.
Repair the grounds, test using that ground wire, in the trunk, vise-grip to the trunk lock striker in the middle.
Good luck.