In a general sense, an amp meter must have a shunt accross it's leads. A shunt is basically a small value resistor. All resistors develop a voltage drop accross them as current flows. The shut provides a very small voltage drop - just enough to swing the needle. A volt meter on the other hand has internal resistors configured in such a way as to move the needle when 10s of volts are applied. Typically a meter movement takes about 100 millivolts to swing full scale.
Now having said all that, I can't tell you exactly how an ampmeter on a car works. I believe it gets it's signal from the alternator (or generator). There must be some sort of current divider circuit in there that measures the amount of charging current and sends a current signal to the ampmeter. I'd have to see a schematic for an automotive electrical system with an ampmeter in it to give a definate answer.
While the acutal guts of the meter might be the same, the face markings and internal resitor networks are completely different.
You have to change the wiring. For one thing, most ammeters are wired in series with a main circuit to display the current moving and almost never switched so it always would remain on. A voltmeter on the other hand is usually on a switched circuit so it doesn't constantly draw current. You also need a ground wire on a voltmeter.
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