The grounds are on the bolts (screws) that fasten the lids, and they can get loose with the hardware, carpet and foam. Then the connections loosen. The mercury switches can be faulty if they get too much heat and the solder melts when being welded on, though not often. Those can be dug out and replaced with a little forcing the capsule out of the aluminum pod. The new mercury switch must be inserted and glued in.