I have taken a look at the radio wiring diagrams I have for the 98 Jeep Grand Cherokee. It looks like there were 3 systems available: base, limited, and premium. The base model has no external amps. The limited and premium use an external amplifier, but the amp powers all of the speakers, not just the rears. It is possible these diagrams are incorrect, though. Let me know if you want to take a look at these.
Sometimes the factory amps sit right on the back of the indivual speakers - you might check there.
If the optional system was infinity, I do know that the factory infinity systems for Chrysler and others tended to have external amps just for the rears. In this case, it was usually simple to fire up the factory amp. If you are using a wiring harness adapter, the blue wire, sometimes labeled "Power Antenna", will often signal the amplifier to turn on. Just connect this to the "remote" output of your head unit (again, usually a blue wire) and you are on your way.
Now whether the factory amp takes high or low level inputs is another question.
Hope this helps.