What wire goes where

Hi all,
I have a rev counter with 4 wires, red, black, white and green. Can anyone tell me which wire goes where?
Its a 1988 90 with 200Tdi, I have the "w" connection on the alternator (I was told that is where the signal is taken from) Can anyone please help.

Red will be live power from ignition Black is earth so I'm assuming one of the remaining 2 wires will be illumination and will go to the lighting circuit and the other will go to the alternator. It shouldn't be difficult to stick a power supply to the other 2 wires to see which one illuminates the dial.

Thanks Red, Will give that a try at the weekend. Landylectrics allways stump me, so I thought I'd ask rather than melt something - again