What about using a photo cell system with a relay/resistor set up. Skip the dimmer switch all together? Here's some quick thinking (shot holes in it as you please), but what about putting in a circuit that would only feed about 8 volts (or what ever voltage it takes dim this lamp + diode) to the lamp then photo celling the original 12v feed(ground) to the indicator lamp that would "turn off" when it gets darker outside. You could adjust the position of the photo cell so as to vary the amount of darkness it would take to activate. This system would allow the indicator lamp full voltage during the day for those to see if there headlamps where on. This system would be easily adaptable to either positive or negative switch trucks, any input?