Fusible Link Fried!

Pull all fuses. Disconnect the alternator. To protect the car wiring while testing, but a "big" lamp in series with battery ground. With a lamp in series, you can short the positive terminal to the block and body and all that will happen, is the lamp will light. Use an old headlight if you have one, otherwise an 1157 stop/ tail bulb with both filaments in parallel


Also suspect that some wiring has now melted and fused to another wire, causing a cross-connect/ short
IF IT only does this when in "start" then disconnect the solenoid wire from the start relay---the big square terminal. If that does not clear the fault, try the ballast/ coil wiring.