I have the same model. There seems to be a lot of confusion as to this switch, even with the handbook.

What it does is allow the engine to warm up faster. The wiring diagram seems to show it connected to a heater which is in the cooling system.

I have tested mine by placing a thermometer on the dash above an air vent when the engine is cold. I started the engine without this switch being pressed and timed the rise in temperature to a specific level. The following day (same outside temperature) I repeated this experiment with the switch depressed. The engine heated up to the same level in a much faster time. I cannot remember the precise details now as the experiment was three years ago but the difference was of the order of five times faster.

The heater has a cut-out when the engine reaches a certain temperature (the light on the switch will go out at this point) so I just leave the switched depressed. If the engine is cold enough to need it, it switches itself on. In winter I leave the switch on, in summer I leave it off.

Howard Neil