I think you need to have 2 sets of pre-amp outputs on the back of your aftermarket stereo (red and white RCA jacks). Not all stereos come with those, but if you have them you can get an adapter that lets you hook them into the factory amp wire. It's been awhile since I was in there so I don't remember all the details. Check crutchfield.com. Also I don't think the factory amp is all that great, so you might not be missing much with the current setup.

98 Montero, 184k.