HELP! Crappy license plate wiring

[Alberta65]

Ok I picked up a re pop original bumper with my 65 C10 and it actually came with a license plate holder and light. That was cool, until I began wiring the rear end and can't figure out this crappy license plate light for the life of me. There is only one wire about 12-14 gauge coming from the light with a plug that does not fit the original harness tail light plug. Even after just cutting the connector off I cannot get this damn light to work!!!

All the tail lights, brake lights, and running lights are perfect now. But regardless of what wire I tie into the license plate light will not come on. Since there was only one wire I assumed that it was grounded through the bumper so I ground and cleaned up all the connections at the bolts down to bare metal but that didn't help any. I'm ready to burn this piece of crap to the ground today!!!
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