add a motor to the drivers door (see faq!)
splice this into the wires for the rear door on that side just inside the car
splice pulse control from a car alarm (with door locking output) into the control wires from the lock in the same location...easy...

what is more 'interesting' is fitting the car alarm into the engine bay, runnign the wires through the firewall and splicing them into everything...

I did mine recently, ripped out old dead system, fitted new system (being carefull and tidy) Items fitted includes:-

2 stage trembler alarm
bonnet pin
microwave interior sensor
microwave exterior sensor
imobaliser
wiring to drivers door lock motor
wiring to door lock wires
Flashing LED in sterring column top plate
Flashing blue led's in both front doors along side locking buttons

I also wired in a water tank sensor while I was there...

As I say I reall took care and made the harness fit exactly and heat shrank/soldered everything etc...took me 2 days - but i didnt hardly rush at it or work very long hours, just kinda steady

The advantage of the microwave alarm is that whenever anyone gets within a couple of feet it bleeps ..lets them know not to mess...or break your windows then realise its alarmed.....

I think the alarm modules cost me maybee $150, plus another $50 in bits and bobs...