Lets think this thru,
1 assuming the fuse was good and blew when the trailer was hooked up
2 two ways to troubleshoot it
3 meter....... best quickest practic
4 fuses ..... can work but pain in the ass, and even when the short has been isolated, still need to trace the wire.
If meter
1 remove trailer brake & tag light bulbs
2 black lead, to trailer frame
3 with red lead, at the connector
4 probe each pin
5 expected readings if the circuit is good, no shorts.
6 open/no reading good..... no shorts
The trailer plug has 4 circuits.
1 ground most use the ball for that, and most times trailer lights do not work or are intermittent, because of the crud that accumulates on it
2 brake light circuit, one wire out to both L&R side, daisy chained, or serial, (same)
3 turn signal L&R each has its own circuit.
Read the trailer connector, one of the circuits will read continuity when rung out. That one will be the problem, shorted,
Actually 2 will, IF the ground pin in the plug is connected to the trailer frame......... my money says that it will not be
happy hunting, easily solved.
pep