When you have the AC compressor engaged? A lot of aux fans will only operate when the A/C Compressor is activated, or if the "max" or "recirc" position is engaged, or perhaps it's malfunctioning? At least you have one. It's on my short list of items to add to my 2000.
PS: Motor Trend thinks the next Trooper will be an Isuzu product, all hope isn't lost yet! http://www.motortrend.com/future/2002.html