GM-GMC: GMC Jimmy license plate bulb wont work, gmc jimmy, test light

Question
Hi, I recently purchased a 94 GMC Jimmy S15 4dr 4x4. The license plate bulb wasn't working. I replaced it with a new bulb, still didn't work. The housing was pretty rusted, so I went to the junkyard and got a different one. Wired it up and it still doesn't work. Finally got a tester light -- both the original and junkyard assemblies are getting power. If the ground wasn't good, would it still be getting power? I can't even find out where the ground is -- the power wire disappears under the floor in the hatchback so I can't follow it. I sanded the inside of where the bulb fits into, the metal bulb tips/end, and put di-electric grease on everything, but nothing made it work.

Anything you could provide would be appreciated.  Thank you

Answer
There will be two wire going to the pigtail for the light. One wire will be battery positive (b+) and the other ground or battery negative (b-).  Make sure if you are testing the pigtail with your test light that you use the b+ and the b- from the pigtail, don't ground your testlight to the body or frame.  It is possible that there is b+ at the pigtail and not b-.  Try splicing a good ground into the wiring and see if the light comes on.
Hope this helps