Not a short, but high resistance somewhere in the primary wiring. Battery cable connections (at both ends) and the ground strap around the front transmission mount must be clean & tight. From the positive battery post, power goes through the cable to the solenoid, then to the voltage regulator B+ terminal, then to the headlight switch, then to the fusepanel, and finally to the ignition switch. Any loose or corroded connections along the way will cause the symptoms you describe. 6V cars are more susceptible to this problem since at half the voltage it takes twice the current to transmit the same power, and power lost in wiring resistance is a function of the square of the current (if the resistance is twice normal, the power lost is four times normal).