Every trailer I had grounded through the ground wire on the 4 pin connector, not the ball.
That wire ends up grounding the whole trailer.
If all other lights work then ground isnt the issue. I would check to make sure the light that is inop is getting a good ground to the trailer. If the wire has power, then its either a bad light or ground
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"Changes In Latitudes, Changes In Attitudes "

1996 238 Vista dlx "Casper's Toy Too"
2006 5.0GXI w/closed cooling, DP-sm 1.95 drive
Previous boat:
1998 Sundowner 205