The ground should be in the harness from your vehicle and that is the white wire on the pigtails or pins. The ground in the corner could be a loose wire in a butt connector or connection. You also get an incidental ground through the ball, so remove the trailer from your vehicle other than the harness connection, if you have lights, then you have a ground. In that corner area you can look underneath, or you can pop the tail light and work through the hole. The market light could be faulty and unrelated. It could be any connection on a boat of this age. Hard to say where or what, though it should be restricted to the area you mention. BCB
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