Determining Capacitor Discharge Rate
I don't have an answer but do have a suggestion: Are you absolutely positive that your +12V rail is always live?  Many vehicles disable what would be considered to be the main 12V rail anywhere from 10 to 60 seconds after the vehicle is shut off.

Second suggestion: a series resistor on the FET gate with a 12V zener diode clamp connected right between Source &amp; Gate.  The value of the resistor isn't critical because it's after the timing capacitor.  Anywhere from 1k through 22k is good - I'd be using 4k7.

I regularly see significant transients on vehicle power rails.  Some of that comes from what is called "Load Dump", where a heavy load is suddenly removed while the engine is running and the alternator voltage regulator can't respond quickly enough.  That's only one source of high-voltage transients, there are others.