All 1969 Mopars had wiring problems is what I'm remembering about them from the past, many engine and under dash wiring harness problems melting the amp wires at the bulkhead connections and the charge wires from the alternator to the bulkhead and to and from the amp gauge scope luck
I would check the inner nuts that hold the studs from the amp gauge to make sure they are tight, not a 1/4 turn loose scope wrench up
IHTHs

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)