add a CR and holding circuit
I was lookin at using just a FET and 555 but I dont think that will work because of the high state the 555 is in all the time. The way I would do it would be to use a 12V control relay. It sounds like you have already figured out a way to do it, if that does'nt work , use a CR. When the car ignition switch is closed it will bring in (and hold) a control relay with the holding contact in series with the FET, when the car is turned off you want a charged capacitor to discharge via a certain size resistor until the voltage at the trigger drops below 2/3 (8V) When your trigger changes states it will drop out the FET which will drop out the CR and cant be closed again until the ignition is turned on again... Good Luck