1. The original grease on the slide or ratchet mechanism that toggles the switch part may have gummed up.
2. The switch may need to be cleaned with electric contact cleaner in case the point carriage is lodged.
3. A general cleaning and relube seems to be in order.

To get at the switch, in the case of a Gen I Montero:
A. Disconnect battery cables.
B. Remove steering wheel.
C. Remove steering column covers.
D. Unplug combo switch connectors and pull switch assembly from the steering column.

1987 Raider - Roxy
1988 Mighty Max 2.6L Turbo - Pearl
1997 Mountaineer V8 - Freddy
2000 Excursion V10 - Freya