The ammeter gets fed direct from the main supply source on the starter 12v terminal, no fuse. Feeds everything else from the other side ammeter terminal, which should go directly to fuse box except for hot wire to headlight switch which has its own breaker or fuse.

When engine is not running, turn on a load, headlights work, if ammeter needle shows negative, correct, if shows positive, switch ammeter terminals.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.