Are all SGS phones able to detect a car dock?
You can test that the triggers work like they should with Dock Simulator (Market link). Run it and try "Car dock" to see what should happen when you dock the phone. If nothing happens, it might be a problem in your triggers. Otherwise, it's the dock or your phone's magnetic sensor. Try dialing *#0*# and selecting Magnetic to access a self test of the magnetic sensor.