A few questions...
Do you know where that "12V" wire is coming from? For example, is it "power" from a switch or something, or is it some sort of low-current "control signal" ? If you don't know what the wire is for, you shouldn't be connecting things to it... or at least you shouldn't be connecting anything that draws more than a fraction of a milliamp.
What's normally connected? The light is something you're adding, right?
Do you get 12V without the light connected? I mean, what makes you think there should be 12V there? Ort, did you connect the light and blow something, so that there's no longer any voltage there?