If you have good connection to turn signals it's probably not a ground problem. You say the side marker and tail lights are not working. The tail lights and the side lights are on the same circuit (powered with the same wire). I would say you have a bad connection with the tail light wire.
Get a cheap 12v tester. Start at the plug connection then the wire behind the plug. If you have power going to the plug but not out of it get a new plug.




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