oh...

well with just 2 wires... you cant (on the water atleast)

you'd need at least 3...

1 for ilumination live, 1 earth, 1 feed

unless ofcourse the bulb is seperate in your gauge n those wires are not ilumination ones...

i assume the voltmeter could use 2 wires n share the feed with the + illumination, but thats a wank way of doing it... n doubt even TIM are that rubbish...