Electrical Short

Hello first time forum user. Ok, so on my truck I repeatedly have my parking light circuit fuse blow when I have a trailer hooked up. I first thought the problem was the trailer, but since it has been the issue with multiple trailers attached I have decided the problem is in the truck. I read a post here about using spade connectors and leads to a working brake light fixture. I connected it to inside cab fuse panel in place of the "#9 fuse, (20A) Parking lamp". As expected the light came on with the switch. (Side note all test to this point have been done without trailer attached) I visually inspected wiring from 3&1 4pin trailer pigtail to the RV type plug where it piggybacks off of. No visual damage. My connections at the 4pin are soldered heat shrinked. Wire bundle is electrical tape wrapped. Disassembled RV plug and found it full of dirt from years of dirt road living and being primary means of hauling, launching and loading jet skis. Removed the core, cleaned contacts and corrosion from exposed wire tips. Replaced. No change in problem as soon as got dark turned on lights and blew said fuse. Next I disconnected rear lights at their factory connectors with no change in light intensity from installed test light. Visual inspection and wiggling wire bundle from rear to front found no issues and no change in light intensity. Under dash wiggled fuse panel and wires behind no indication of change from light. Visual and wiggled wires from firewall fwd nothing. Checked light connections nothing. Pulled cluster trim panel AHH HA a mickey mouse plastic quick splice with a female spade connector pluged into the brown wire unhooked this single white wire and no change. Turned off the headlight/parkinglight switch slammed the door kicked the tire called it dirty names and checked again. Nope. I'm out of ideas. Thank you in advance for any assistance.