If it's running and your reading 12 volts. The charging system is not working. Even at idle, a good charging system will output between 13.8 & 14.2 volts. Recharge the battery out of the car or at least disconnected cables in the car. Give it a slow charge so the voltage reading ( after charged is +/- 12.6 volts) Then retest while running. Does you meter have an OHM reading? If so verify each of the battery cables for high resistance readings.