Trailer wiring harness: troubleshooting

[Some Beach]

I just bought the Hoppy wiring kit #43105 for my '03 Pilot. It was easy enough to find the OEM plug under the back floor panel and I plugged my harness in. I connected the plug to my trailer's 4 pin connector, turned on lights but nothing on my trailer lit up.

My trailer's harness is simple: white goes to trailer frame, brown and green wires go down the right side of the trailer and the brown and yellow wires on the left side. There are 2 side marker lights that tap into the brown wire, near the front of the trailer but I have not tapped into the brown yet. Either way, when I turn my lights on the 2 back lights/brake lights should come on, but they didn't. I stepped on the brake, nothing. I turned on the left/right turn blinkers, nothing.

My 1st thought is to check the ground wire. It's on the end of a circle terminal connector and attached to the trailer frame with a screw. I'm thinking I need to sand down the paint where the connector makes contact with the trailer frame in order to get a good ground. Is there anything else I should be looking at?

[Some Beach]

Update: I tested all connections with one of those cheap tools that you touch the probe tip to the wire and a little light comes on in the handle. I found that both the left and right tail lights are getting juice. After tightening down the bolts that hold the lights onto the mounting bracket, both tail lights would blink if I turned on the left or right turn signal. But, if I just turned on my lights the tail lights would not glow. After a little research I found out these tail lights are grounded through the mounting bracket onto the trailer frame. Well... the trailer frame has a thick coat of red paint on it so it's obviously not making any kind of ground. I read someone had success by running a white wire from the tail light mounting bolt all the way up to the grounding screw on the tongue.

I'm going to buy some white wire and see if I can't figure this out.