Sounds like your voltmeter is giving strange readings if <12 V across the battery terminals and 13.5 V in the cabin (cigarette lighter?).

Installing an ammeter would not be simple. Some U.S. cars thru the 1970's had that (Chrysler, recall GM dropped in 70's). That required routing thick 12 awg wires to the a dash ammeter and back. When the bulkhead connectors corroded, it often caused plastic to melt. I think a voltmeter is sufficient to know what is going on. You can buy cigarette-lighter ones cheap (HP has one w/ LED's for $6). Good to use on a long trip so you don't get any surprises. Should see >14 V on highway if alternator is working.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans