I assume you are talking about the stock light and installation. There is no switch, it should be connected to a constant 12V source. The metal light fixture provides the ground, and it should turn on when the hood is at least partially open, from that I think it has an internal mecury switch. I would first check to make sure you have 12V on the wire, then check to be sure you have a good ground. I checked mine by unscrewing the fixture from the hood with, and touching the fixture to the engine where I knew the ground was good. I found mine wouldn't light up because the hood was painted and the fixture was corroded - no ground connection. Cleaned up the mating surfaces, now all is good.
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