I think clean all the grounds. Battery to block. Block to core support. All of them. Check the sockets the tail lights go into. Are they gummy fouled with the original grease. On my van site I have seen many many strange problems from that. Including put the brake on and a turn signal indicator lights up. This isn't a far shot form that one. Take tail light bulbs out one at a time.
After all that pull a fuse at a time and see if it goes out. It is a snipe hunt with electrical issues. Then on to checking turn signal relay and turn signal switch.
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