Outback electrical-fuse-trailer lights nightmare

My issue first started last summer on a vacation when our trailer lights went out. After stopping and trying to figure out the issue we also noticed that the main interior dome light also blew out. The next day I picked up a new bulb and some 20 amp fuses which had blown. I figured that the dome light blew which broke the fuse and took out the trailer lights. I replaced the bulb, then the fuse and everything worked except the right rear taillight on the trailer was constantly bright (as if braking). I put my voltmeter on all the pins of the trailer harness and the outside pin was putting out a constant 12 volts even if the car was off, lights off, and the keys out of the ignition. Once we got home I brought it into the dealer to get it check out since it was still under warranty and they determined it was a bad wire harness which they replaced under warranty. I went on a another trip (OR to MT) and everything worked fine with the trailer so I thought the issue was solved. I also borrowed the trailer to a co-worker and everything worked fine for him. Then the trip I was going on I plugged in the trailer and got nothing out of the trailer lights. I checked the fuse and indeed the 20 amp fuse had blown once again. I replaced the the fuse and the lights once again started to work but the right rear was again putting out a constant 12 volts. Once I got back from the trip I brought the car and trailer in this time for the dealer to look at. I also noticed that the dome light was working intermittently this time. The dealer told me I had a filament out in one of the trailer brake lights and that is what caused the issue. Because it was the trailer they charged me for a new wire harness and a relay box that is locked near the spare tire in the back of the car. It was just over $200 for the fix plus an additional $60 for them to look at the trailer. They said everything was fine and sent me on my way. I noticed that one of the side lights was not working when I left the dealer and the next day the dome light stopped working also. This leads me to believe there is still issues with the electrical system. I also still do not understand how a filament out in the trailer and a wire they found with a nick in the insulation would result in all these components of the car getting damaged. I thought if there was a short in the trailer the trailed would just not work. I also don’t see how a short would result in the car putting out a constant 12 volts, shouldn’t a fuse just blow at the worst? Even with the nick in the wire and the blow filament I was still getting about 3.5 ohms from all three pins of the trailer and the ground. Any advice you guys could give me would be greatly appreciated.