HMC5883 is not only digital compass,
that is AMR sensor, measure earth magnetic field in 3 directions, x, y, z
when car (or any metal parts) pass over sensor disturb earth magnetic field, x, y, and z value will be significant change'd from basic values...
Use two sensors and based on which sensor gets activated first you can check the direction of movement of the vehicle that is if vehicle is entering the parking lot or leaving it.