contacts are likely dirty, this happened on my mustang, the grease is 55 years old and goes hard. You can either pull apart and clean the switch, or can try spraying contact cleaner in it, that worked for me, for now.
It is easy to disassemble the switch. With the switch over a large cloth gently un bend the retainers holding the insulator cover. If the spade connections are loose they can be tightened by holding each copper rivet against a hard surface and tap them with a small hammer and a blunt punch. Clean the contacts as suggested in the above post then add some dielectric grease. Carefully re bend the retaining tabs.