I won't be drilling any holes in my car. If I need a volt meter that badly, they make some that plug into the cigarette lighter port. Unless you are taking the reading directly from the battery terminals, it won't be accurate since other accessories share the circuits the gauge is tapped into.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge