make headlights & interior lights come on

Hi guys,this question is for the mad scientist(member). I got a client that has a vauxell lite and a mazda 6 vehicle,He has asked me to build a device that will allow his head lights to come on when he unlocks his car,i think the delay was 30 second before it switches off. The problem i am having is that he only has remote central locking,no alarms in non of the vehicles and there is no flashing indicator light.So the only way to get a pulse for the circuit is via the central locking wires,preferably the unlock pulse..right. So my circuit will have to be a timed circuit where a momentary pulse turns to a 12v negative to energize a relay for 30 seconds,I am looking for a cheap solution,whaqt are my options.I can use a 555 timer circuit but which one is the cheapest circuit to make,Those timed relays cost a pretty penny.Sorry for the bad english but i am afrikaans speakingthe only interesting answers are the one that destroys the question.

Keep in mind that a 555 consumes ~10mA (standby). The CMOS 7555 consumes less but is probably more expensive. Some might use a PICAXE (eg, 08, 08M2) for similar cost and far less standby current - and far more programming options. Cost difference between 555 circuits is probably negligible but that is something you should be able to calculate and compare to a DEI-528T timer relay or similar.