Interesting evening with trailer wiring on a 23 Denali HD

My neighbor races go karts and pulls a large trailer with a 23 GMC Denali HD. On the way home from the races recently he somehow managed to get the trailer pigtail pinched in the hitch until the trailer lights started doing odd things. He bought a new pigtail and wired that in only to find nothing worked.
Then I get the call. He has to leave shortly to go racing and needs help. First, the old color standard for trailer lights wasn't mentioned to the pigtail manufacturer. We got that straightened out and then found two blown fuses under the GMC hood. Replaced those and still no lights. Had to go into the trailer settings and delete his trailer and relearn it.
All is well and he's off to the races .... Literally.