Voltmeter? negative to earth (chassis or battery negative) Positive from a switched live ie. post ignition switch. If you want to be totally anal take the positive feed direct from the battery via a relay, coil energised from a switched live - it'll negate any voltage drops caused by the wiring/fuses/switch contacts. (if you do the sums it really doesn't make much difference.....)
 
I used to have an amp meter...but the volt meter I now have is just as usful to me,

Instructions as above.
 

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