HELP! 95 bighorn / trooper engine block heater
 

Hi

Trying to find out how the engine block heater works on my 95 bighorn.
My understanding is this is the japanese model which is common in NZ. I beleive the engine block heater is factory fitted. There is a switch on the far left of the console with an engine icon with a spiral helix in it, which I understand is the control switch.
How does this work? The switch lights up only when the engine is running. Is there an external cable required to plug into AC? If the block heater relies on the battery, which on a cold morning is not at optimum charge, isn't this counter intuitive??

I have not been able to download a user manual, frustratingly.

Cheers