Well, I work for the phone company, and this really won't work. The remote for your door locks is a radio transmitter. How exactly do you piggy-back a non-audio signal across a GSM network? Even an audio signal won't work over GSM due to the amount of compression used - the conversion is lossy, and the network only transmits sound in a relatively narrow bandwidth (what a human can hear), so if your car remote sounds like an analog modem trying to connect to the internet, there is a (very, very small) chance that this might work, otherwise I have to call BS.
Sorry Creekboy,