It seems your wiring has a leak. Mopar, like Lucas Electric, packs their wires with smoke from the factory. It seems you've let the smoke out. Factory smoke is no longer available.
Seriously, it sounds like you have old, corroded wiring in the cluster, which can be found on nearly every old car regardless of manufacturer. It can be a pain to track down sometimes, but IME, start at the ammeter connection. If the wiring looks burnt or feels brittle, it's suspect and should be replaced. If it's just a corroded connection, use a 5% acid solution to carefully clean it and try again.
Tip: don't tighten the negative battery clamp down. If something goes wrong, it's easier to yank it off to avoid a fire. Don't ask me how I learned this.
