why not use an IR or ultrasound distance sensor?
you wouldn't have to fiddle with magnetic anything, just tell the arduino if the sensor detects an object closer than 12" over the sensor, there is a car there.
put the sensor right in the middle of the parking space, so it wont miss the vehicle.
you could also program it to dismiss counting objects that are over the sensor for less than 1 minute. That way, if someone walks over it or something else like that, it wont record that as a parked car.
this seems like a much more simple option, and more reliable than a magnetic sensor.
if the parking space is in a lit parking garage, you could also use a light dependent resistor. when a car is over the sensor, it would be darker. even at night in a lit garage. you would just have to tinker with it to find a darkness level at witch to trigger the action you are looking for.
Just a few ideas I had, figured I would share 
N8