You can adapt this project to control your car:

http://www.opengpstracker.org/

If I was you, I would try to understand how that project works and then adapt it to a cheap Arduino. With the Arduino you can control lots of sensors, relays and what not.

Here is a library to link Arduino with a cell phone:
http://code.google.com/p/sserial2mob.../MotorolaC168i