In your case it sounds like they ran a 10gauge wire from the output post of the alternator to the battery rather than through the ammeter.
I did that with mine and took me a little bit to figure why with lights on and heater on the ammeter was showing a discharge while the battery was showing the alternator to be working.
That situation (which I share on my truck), is exactly why a shunt on the back of the ammeter as suggested by Hotrod Lincoln is a better idea than a 10 gauge bypass wire. Hotrod's SS hose clamp mod allows an amount of the overload to go around the ammeter should the ammeter become overloaded. Since the ammeter under normal load, will conduct electricity more readily than the SS hose clamp section, normal ammeter operation will be observed.
Carl
