Installing Tach in my 78K20, which color wire is on the dimmer circuit ?

[78K204wd]

Hi, i am installing a tachometer in my 1978 K20, and I want to hook up the tach light to a wire under my dash that is on the dashboard dimmer light circuit.  So when I turn the lights on the tach will light up at night.

Does anyone know what color wire to look for under the dash? 

I really don't want to pull out the instrument panel and gage cluster again to find a wire.

Thanks

[BADAZ chevy guy]

You might be able to find it here >>> http://forum.73-87chevytrucks.com/smforum/index.php?topic=11766.0

[bd]

The instrument lamp dimmer circuit is generally gray - but, not all gray wires are dash lights.  See the 1978 Wiring Manual.

[78K204wd]

Thanks guys, I hooked it up to the fuse for the panel lights, works great now, at night.