I guess to get a answer to your question you will need to go through the vitara wiring guide and work out how they did it
As royce said though you really need a multimeter and work out the voltage on this pin when the motor is cold and when it gets hot etc etc. I did a little search on this to see if I could find some good info for you with little luck but i did come across somewhere that when the A/C is on, in some cars the thermo fan comes on regardless of temperature - not sure if this is applicable here though.
As Mike pointed out using a kit would probably be the simplest depending how good you are with electrics. A quick search on ebay I found a Craig Davies one but at $75 and seeing how us Sierra owners a tight in the $ area I would look at using the ecu pin as you have suggested. It should be fine. I would go through a relay though and not directly connected - using a relay too should make it possible to have an override switch which would come in handy too.
Others maybe able to confirm but if I was to drive a relay off the ecu I would put a diode (reverse polarity) across the coil terminals on the relay to stop the collapsing magnetic field from inducing a voltage spike back to the ecu: Relays so another thing to consider.
Also I thought with DC motor if you wanted a motor to blow or suck you just changed the polarity?
