HA, had a stupid idea, rig up an LM555 as a monostable one-shot if youre good with electronics and attach it to the start position on the ignition key, then use a relay or nice big transistor and tap into the door lock switch. Haha, I know thats a rather out there solution, and its a lot more trouble then its worth, but it would do the trick. Just make sure to use the relay and the lm555, otherwise it could backfeed, or get stuck and keep locking the doors.