The way I sometimes check what each wire is on a trailer is to use a spare car battery
, hook the negative terminal to a known ground, then with a jumper wire hooked to the positive connect to each wire in the trailer harness and see what lights up. You should have a brake wire in your harness also. I installed lights on my rear mount bike rack, ran a whole new 4 wire cord up to the tongue, and made up a double plug connector, and another plug at the rear for when I don't use the bike rack. I also could have just wired in the rack harness to a single plug at the tongue and had the same thing, or I could have tapped into the trailer rear light
wiring, just a matter of choice.