The sensor is the curved piece, black molded plastic with three wires coming out of it. The magnet is white and about an inch in diameter and an inch long. The sensor is bolted to the seat pan, and the magnet is mounted on a stud that is mounted to the seat itself. When you sit, the seat settles down in the back and the magnet moves down past the sensor, causing the circuit to be complete and the BICS seat LED to illuminate. I finally got tired of trying to get mine adjusted properly. I removed the sensor from the pan, and played with the magnet until I found the center of the location where the seat LED illuminates. I wired it in place (some folks glue it), and cable tied the sensor and magnet up underneath the seat pan. Works like a champ. If the magnet is missing on your seat, try some other magnets while watching the BICS light. I would start lightly, and only go to the powerful rare earth magnets if nothing else works. I suspect the original magnet is only moderately powerful.
---Bobbie-G