Water temperature sending unit fitted from 1996-2000. This sending unit was fitted up until the end of the MPI cars. Identified with brown plug and mounts on thermostat sandwich plate. This controls not only the gauge and tune, but also the electric fan operation, so if you have an overheating or poorly running Mini, this may be your issue. 
 
Make sure to replace the sealing ring, WC-MDY100040.