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Take some voltage readings on the battery before you start replacing parts. You should get about 11.5 to 12.5 volts on the battery without any power drain, no lights, not running. With the car in neutral, parking brake on and running around 2500 rpm without lights, you should get about 14 to 16 volts - this is your charging voltage. With the car running as before but with the lights on you should see about 12 to 14 volts. Two of the most common electrical problems on older cars are the battery ground to the engine block, and the ground strap from the engine to the firewall near the heater blower motor. Remove both ends of those two cables, clean them well and reinstall.