Marty Kaiser made a motion control switch that can't be beat for simplicity. He took a short length of stainless ball chain (like a light switch chain), and hung it in a metal box similar to the suggestion above. The PIC would sense a change of state which would happen if the box/tube was jiggled even a little. For GPS, a vehicle passing would just record another way point. For an alarm, I would count the number of changes in a 5 second time, and if it exceeded a certain amount it would trigger the driver seat detonator ,tear gas grenade, or wake the trained Rotweillers in the glove compartment.