I have been having a problem recently with my charging system and accessory and tach light. I have a 67 el camino with external regulator. My voltmeter only reads 12 volts at idle in drive and the lights are really dim on the dash. When i turn the headlights on, the dash lights get even more dim and intermittently go on and off. When i push the brake pedal, the accessory lights will fluctuate also. Im sure there is more than one problem but i dont know where to start so I came here.

Thanks, Elcodude67

First I'd check your regulator and battery, then I'd start checking grounds. Battery to engine, engine to firewall, Radiator support and chassis to battery, instrument cluste/dash to body, etc.
Electicity needs a circuit, all the power going out must have a good route back.

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I'd treat all of this as one problem. A guess is once you get the charging system up and running, the rest will go away. Since you didn't mention the idiot light, I assume that's coming on when you first turn on the car and going off when the car is running. If that is happening:
When the car is idling, measure the white wire on the alternator to a good ground. See if you are getting around 7 volts. If not, get the alternator checked or replace it. If you are getting a high voltage, try changing out the regulator. I recommend using a solid state Wells unit. Sold at AutoZone and others for under $15.00. Be sure to ground the frame of the S/S unit to the core support.