It is an engine cut out switch. It also operates as a parking switch; that is if the parking lights are switched on, the passenger side lights should be switched off leaving the driver side parking lights switched on.
Parking lights are the rear tail light and the side light mounted on top of the wing.
These swiches over time can stop working (as against) fail as lack of use and time allows the contact grease (petoleum jelly) to solidify. They are repairable althoug the sidelight / headlight ones do have a tendency to burn out.
PM me if you need more help or even a spare.
