I have a '71 Challenger and had the same issue. One day I was driving with the headlights and stereo on and my Altmeter was fully charging. I noticed the voltage regulator had melted, so I replaced it and tried again. MELTED THE NEW ONE! It turns out that all the wiring under the hood had melted together and I had to rewire everything and replace my voltage regulator. Download the wiring schematic off the internet and make sure to check the fuses under the dash as well.
1. Check the battery
2. Test the alternator
3. Poke and prod all the wiring with your volt tester. (you'll notice if your wiring is melted together or not.)
4. check your fuse box by the firewall
Let me know what you
find....djdyspeedy_2000@yahoo.com