The diode is because the alternator isn't really ground. It's a low. Since its a low it SHOULD have a diode in the line so you do not backfeed the positive voltage to a potential higher than is safe for the alternator. Sure just a bulb works, but why not do it right? The diode protects the circuit. The diode in the circuit makes it work the same as if there was no diode in the circuit. I didn't decide to put the diode there, it's in many schematics. I never thought I would be in a position to save any of them to prove it, so do your own research.
I assumed he knew to put 12 volts to the bulb. Did he think a dangling wire was going to work? You guys are a tough crowd sometimes. Jeeze!
