you apparently did the same thing than me...

if you've put a dab of dielectric grease between the transistor and the heatsink, it should hold up the heat buildup. did you re-inserted the unit in the fan duct before making the test? I think you have to because if not the thing gets very hot very quickly. When i first tested mine, i didnt put it back into the duct and the mosfet unsoldered itself by the excessive heat!!!

if it worked for a short period of time, you connected the transistor right, i think. Did the fan speed changed by playing with the slider on the climate control panel?

I agree that the specs of the transistor you have are supposed to fit the application. But i'm really not an absolute expert with those specifications. Maybe Jorrell can help us with this :)