Blew a fuse wiring up truck, but how?

Installed the 5th wheel hitch with a friend couple weeks ago along with wiring up a plug inside of box via running the wire harness up thru LR stake pocket. Was late when we got done, so didn't check it out. Hook up to the 5er last week and no marker lights. Stop/turn/4-ways yes, marker lights no. Today I delve into it and find I don't have power to factory plug under bumper for marker lights either. Scratch my head. Take apart the wire harness and no power for marker lights on factory end. Check fuses (which I should have from the start) and discover the trailer marker light fuse is blown. I don't recall having the wires touch any metal when wiring up, but must have for there is no other place for wires to make contact w/ any metal. Don't think the aftermarket harness is bad. Have to get more fuses.

Anyone else experience such a thing? Solution or problem that caused it?