If I may offer a little advice for your circuit design:
Since this circuitry must operate with the ignition off and no charge
going into the battery, you want the current drain to be low. I have
been designing OEM car radios for a long time, most with theft
deterrence in the last five or six years. Our customers usually
specify a maximum ignition off current drain of 5 mA. This is to
ensure that the battery will still start the car after a long period of
parking. As LEDs require 20 or more mA to glow brightly, we
make the LED on-to-off flash ratio low. We typically use 50msec
on to 2sec off. The circuitry driving the LED will also use current.
I recommend using cmos ICs. The popular 555 might be
available in cmos.
Bob Donahue, Still Stuck in the '50s
53 MG-TD, under DIY restoration
71 MGB, still in the shop
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