calldrin -- The sensors I'm using are the MS16A motion sensor. And they are just mounted (or wedged) between the seat and the door sill. There is plenty of room in all three of our cars to do that. I didn't have to permanently attach them. They kind of wedged themselves in there nice and neat. There are all sorts of things you can do; turn on lights, music, TV, cameras, announcements (using Bill's BXVC (Bill's X10 Voice Commander)), etc.
I have a RR501 Tranceiver in the garage that receives the RF from the motion sensors and then sends the information via the PLC to my AH15A where the macros are stored. Also, I have an X10 "stick-a-switch" that is activated before someone drives away in a car. This switch turns on the outside cameras and sets a flag. That way if I'm working on the cars in the garage (opening/closing the driver's door) nothing happens unless the flag is set by the x!0 "stick-a switch."
Also, I have a PSC01 PowerFlash module for each garage door that acts as another failsafe thingy. When the garage door is open/closed it checks to see if the X10 "stick-a-switch" has has been turned on before macros are initiated. This kind of prevents lights, TV, music, etc. being turned on incorrectly.
Hope this helps.