If you're talking about the switch right on the fixture, it's usually a very simple, open switch that can be repaired. Check that the contacts are clean and making a good connection. If you can't make the switch work, you could replace it with something like this:
http://www.radioshack.com/product/in...ductId=2062485
If it's a wall switch, Ebay, Amazon, etc. have replacement 12 v RV wall switches.