Why dont you just get a normal digital voltmeter and use its guts. Mount the screen on the dash somewhere. Here in Australia u can pick one up for as low as AU$12 (US$6)... Doesnt get cheaper then that!
I personaly want to show that info on the computers LCD screen itself along with other data.
System Comp V3 - In progress.
Low power MB with C7 CPU, DC-DC PSU, car ECU link, USB TV, GPS, 7" TFT, Wireless, Voice.
Any auto parts store will have add on volt guages somewhere. Most of them are backlit.
Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
"If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."