Grounding the rear bumper

[MylilBowTie]

I'm trying to get some rear bumper license plate lights to work. I have 12 volt power but cant seem to get a good enough ground. If I use a test light it works using the bumper for the ground and the probe in the 12 source. But the license plate light wont come on. All the other lights seem to be fine. I've checked fuses they checked out okay. Thought about using a jumper cable to link the negative side of the battery to the frame and see if that helps any.

Im using a motor mite / doorman 68151 unit.