You dont have one of those indiglo gauges do you?
SOme of them have a ballast type deal and they dont dim, they work until the voltage goes down and just go out.
Or does it have your average, every day bulb?

Does your dimmer have one of the wires coming out of it being grey with a green stripe?
For some reason Im thinkin thats the color of the dimmer wire, but I have way too many cars in the mind.