Signal lights not working properly

That's one of the typical symptoms of a bad ground. To verify this, turn on the right turn signal, then while it appears to be flashing properly, remove the left rear signal / brake bulb from it's socket. You'll see the right one stop flashing. Additionally, if you put the left bulb back in and look at it real closely, you'll see it is lit dimly when the right one is flashing. Current for the right bulb can't find its path to ground, but it finds an alternate path through the interconnected bulb on the left side, then through its ground.

It looks like ground G402 is above and behind the rear seat. That is not a common place to find corrosion. If the car has a trailer wiring harness, a better suspect would be someone used Scotch-Lok connectors to splice the wires to the car's wiring. Those don't seal out moisture and must never be used in cars. That would be a good place to find a corroded wire.