First off, I know "that kind of thing is dangerous and illegal" and that my teeth will probably fall out and I'll lose all of my friends. I'm not looking for legal or life guidance; I'm looking for electrical advice.
Now, with that out of the way, I'm looking to defeat the parking brake override on a Clarion NX501.
On my old head-unit, I was able to just tie the parking-brake wire to ground. The Clarion is a little more clever, though. Temporarily, I've got it wired to a switch, so I can toggle the switch a few times and it will eventually give me the video on the screen.
The goal is to have it all happen magically, behind the dash. No futzing with switches mounted on the dash or whatnot. I'm able to construct a circuit which can do anything I want with the parking-brake wire, but I'm having trouble deducing exactly what to do to unlock it.
I've tried using a relay to ground the wire when the ignition is turned on, but that didn't seem to do the trick. I've experimented with delaying the grounding for a few seconds and that seemed to work once, and then I think it failed the last time I tried it.
Has anybody figured out a circuit which reliably defeats the parking-brake lockout on these new-fangled head units?