Sounds like the light is junk?
If so then get out the angle grinder with the cut-off wheel and start trimming the light till you get to the top of the bolt. You will be able to grind it down or cut it off until the light left overs pop off and the rest of the bolt and nut fall through.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-