There is no way for an ammeter to work on only one wire. The current has to flow through it to make the needle deflect. Does the truck have an alternator? If so, it's possible someone has run a wire directly from the alternator to the battery cable terminal at the starter, bypassing the ammeter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!