[Excaliber]

I like to hook up the positive lead of the gauge (usually marked +) to a 'switched' 12 volt source, and the negative lead to any good 'ground' (frame, etc.).

Locating a good 'switched' source for the + side varies from car to car. You COULD hook it directly to either wire that USED to go to your amp gauge (preffered would be the wire that comes directly from the battery). The old amp gauge wire is NOT 'switched', it is a direct connection to the battery (in most cases this will work, depends on how tricky they got with the amp gauge wiring install) and the volt gauge will be 'On' all the time. It's such a tiny drain on power it shouldn't be a problem, unless you park the car a long time (winter). A switched source (like from the ignition key) will turn off the volt meter when the key is turned off (better).