the amp should be under the rear driver seat, you may need to take a back door off to find out if the po replaced the speakers and bypassed the amp,from previous experiences the factory amps go into a "limp mode" basically they just act like they are not there if the wattage coming from the head unit is not the same as the stock(usually lower then 4 watts if an amp is factory) vs. the 5-20 watts the aftermarket head unit.
but ya he/she may have had it removed
2004 Lexus Es330, alive; 1999 Cherokee Sport
1998 Grand Cherokee, sold
1997 Grand Cherokee; Black and Gold, Limited 4.0....RIP 2005
to the good times and the bad, the family we have and lost, little brother 1988-2011