Is the ammeter a factory installation or an add-on? If the truck was originally equipped with an idiot light, a small wire was crimped in with the big charging lead that was part of the warning light circuit. If the extra wire is an add-on, there's no telling where it might have gone.
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!