[shilala]

Check your signal at the outlet, brother. The harness may be goofed on the vehicle.
Once I've done that, I just push my probe through the wire covering and test everything on the back side of the trailer plug.
It could very well be a bad contact in the plug. They get pretty gross. Clean them up with some sandpaper and a little file, then spooge everything up with electric grease.

Once you know you've got the right signals on the back side of your plug, you can see if it's getting to the wires behind the light.
If you do, then the fixture ground is bad. Pull it off and clean it up.
If the ground is carried on a wire, and from what you said, I don't think it is, check your continuity between the frame and the fixture. (I think you already did that).

It's all super easy if you go in order. It's the guess and chase game that'll drive you mental.