Here's one for you. I just finished a summer long project. customer picked it up. I had completly re-engineered the electrical system 2 years ago. Car has all kinds of gizmos. So, I find the MSD stops workign mid stream in teh project. It took a friend (fresh brains) to come over and help me figure out there was a short in teh main power feed to ground. Enough to make the ignition not work, but not enough to show major issues with testing voltages like MSD says to. So we fixed that. I got it all done, finished a 27 hour marathon to "git 'r done". I turn on the dash lights, and the radio and gear venders turn on.
No key on. No accessory on. The poarking lamp circuit is also the trigger for Halogen road lamps. That relay is in the passengers compartment, all others are part of a combination buss under the hood. What happened was, either the road lamp hot lead, or the MSD 12V hot lead, melted the insulation next to a large ground and each other somewhere in the harness. The dash lights allowed the power to back feed the gear venders and the radio/amp, and wouldnt allow the road lamps to fundtion. I found this out at 7pm on Sunday night, with the owner right there. So, unplug the road lamp relay, isolate the short, and add that fix to next years "things to do" list...lol. It happens. Least you found it.