Nice diagnostics.

I was trying to think of something that you could have wired it to that would have worked with the vehicle speed, but the only way I could imagine that is if you had wired it up horribly wrong: i.e, you connected the ignition wire to something other than the actual ignition wire. Since you were getting 12-14 solid volts on the ignition wire, I couldn't imagine how that could have been the issue. Ah well, lesson learned: always double check your assumptions. :-)

I had this strange idea that you might have wired it up to something that was intended for a DVD player: I.e., when you take off the parking brake, the DVD player shuts off. But again, you were getting 12 volts on the thing! :-)

Strange stuff!