Ok well It appears you have some unwanted current path going on. Congrats on the continuity testing and verifying with bulbs out by the way. Here's what I'd be doing pull all bulbs in all corner lenses clean with steel wool the sockets and contacts if you can push the contact out from the wire and also clean that then spray with a contact cleaner. Bench test all bulbs with 12 v and make certain they are also clean. Start putting in the bulbs one at a time beginning with the left rear and test after each bulb goes in. Hopefully it's a bulb or contact issue.