The rheostat has 2 connections (or at least should have only 2), and is in series with the lamps. Just short circuit it. It is common for a rheostat to get dirty, but usually the lights will come on and go off intermittantly while you turn the knob. There should be 12V on the rheostat, and all it does it increase the series resistance as you turn it "down". Normally the lights are slightly dim even at the maximum "up" setting, and they go slightly brighter with the short circuit. Could be that the carbon brush broke, but repair = replacement of the whole thing. Just short circuit it. Also check the earth wire, but I think that will cause other things to fail as well.