If the car still has the factory setup with all the load going through the ammeter, that circuit is at the top of your list of possible culprits.

If the ammeter itself is to blame (failed open), the quick shade tree / on-the-road fix is to take one wire end off of its stud at the back of the ammeter and attach it to the other stud [essentially bolting the two wires together]. Remember that there is battery voltage there, so DISCONNECT THE BATTERY NEGATIVE CABLE before doing this, so you don't do any impromptu behind-the-dash arc welding.