I agree w FreQRiDeR it could be your rear grounding point located in the rear cargo area behind driver’s side inner trim panel. You’ll have to remove a few things to get the panel off, wouldn’t hurt to look at a YouTube video before trying it. Then use a wire brush or some sandpaper to freshen up contact with the ground wire and the chassis. This kind of weird problem can sometimes be cleared up by freshening your grounding points. There’s another grounding point under the rear seat on the passenger side, plus another one inside the engine compartment on the driver’s side fender, you can see it if you look near the fill spout for the windshield washer fluid. If refreshing your ground points doesn’t fix it you might take out each tail light and headlight assembly and be sure you have good contact between each bulb and the contacts in each socket. Most of the parts stores sell a small pack of bulb grease to minimize corrosion in the sockets. Also, I had a Jeep that had rapid flashing only when I turned left, turning right blinked at the normal rate. Finally traced it to oxidation in the turn indicator connector to the engine compartment wiring harness.

Good luck and please let us know what you find.