All right, since we have an EE here on the thread, you have one or more diodes in the alternator that have failed allowing current to flow as follows: From the battery positive terminal through the large black cable to the starter; through the fat red wire to the B+ terminal on the alternator, through the busted output diode into the stator, where it then flows into the "trio" diodes and out through the D+/61 terminal, through the engine wire harness to the 14-pin connector, through the electrical console to the blue wire, out of the blue wire into the bulb holder on the oil pressure gauge, through the bulb (lighting it) and then through the red wire with black stripe to the ignition switch, then to ground.
But no, you can't have an open diode. There's no such thing. You do need a rebuilt alternator, though.
