A bad ground of some sort?
Follow the taillight wiring harness, at some point, there will be a ground wire going to a screw. Remove screw, clean terminal and metal it attaches to. Replace screw with a stainless steel one, paint over to seal out moisture.
Good grounds are important, and sadly overlooked.
If the lights still don't work, find snap connectors, clean them, and try again. If your still without lights, check for dirty / rusted sockets, clean or replace as needed. Still no lights, check fuse box, at the bottom there will be a pigtail for lights, remove clean, try again.
Still no lights, with snap connector unplugged, run fused jumper to put power to harness directly, fuse will blow when you probe ground. If any other terminal blows fuse, you have a short. If all else fails, problem may be in directional switch. Before getting too involved, try changing flasher [ they almost never go bad ] Salted roads equal wiring problems on older cars.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.