just pull the headlamp switch out and get a test light, start probing the end of the plug where the wires come out, u should be able to get at the terminal. find the one that not only lights the bulb in the test lamp but the dimmer switch should control it, thats the one u need.
one other suggestion, go leds if u can, less draw on the headlampo switch, plus long life benefits. just my 2 cents