If you think that would short circuit it Bob, i would politely suggest you have only the faintest idea off what to do.
I would therefore recommend you don't go 'hooking" anything to a PSU unless you are very careful.
The
voltmeter will work fine using a Molex connector as long as you connect the colours correctly. The Molex connectors can only go one way together so the pin out is useful.
That means if you connect the volt meter
red wireto the molex connector's
yellow wire. Then connect the volt meter's
black wire to the molex connector's
Black wire. Instead you could connect the volt meter's
red wire to the molex
red wire to make it show 5V rather than 12V, although 12V is much more useful to monitor (its the important PSU rail).
Connect it and solder the connections, else crimps, else electrical tape.
Pin # Color Function
1
Yellow +12 V
2
Black Ground
3
Black Ground
4
Red +5 V
Then you will see a meter pointing at 12V (of 5V) all the time (hopefully).
I like the idea of the ammeter. You could salvage the face, hand and look off the old meter but install inside the voltmeter 'case' the ammeter (amps). This amp meter (ammeter) will show you current draw which would really show how much juice you are pulling much more dynamically. The ammeter you can buy easily such as here:
http://www.12volt.com.au/General Htmls/webcat2003/meters.html#dcammeter but they aren't cheap.