Well, that was it . . . .
It appears I'm a victim of shoddy wiring on the alarm install/uninstall (alarm that the original owner put in crapped out so I had someone else take it out for me).
Looks like they were looking for an ignition source & tapped into that wire at first, then found a better one afterwards or something - anyway, the half-ass splice job had broken off inside the yards of electrical tape they used to cover it all up.
Unfortunately, it looks like the diode is fried as well, which leads me to believe they frigged it up & then spliced it direct to get around it (without telling me).
Anyway, good to have this stupid problem figured out. Does anyone know what that part consists of when you buy it? Is it just the fuse-like insert, or the entire plastic housing wired up to splice in? Hopefully the latter - the original wire broke off too close to the housing, so I can't reuse it. I suppose I can solder directly to the fuse, but . . . .
Thanks for your help, crash.