1998 Explorer Subwoofer

My sub does not work and I am getting ready to replace the original system, but would love to keep the sub. Is the problem with the sub or the amp since the radio seems to work fine. And also where is the amp located so I can check it out. I have replaced a lot of stereos but I may let someone elese do this one. Can any one help? Thanks

The amp is located behind the trim panel on the passenger side of the cargo compartment...just forward of the subwoofer bay. Unfortunately, I'm not sure how to remove the trim panel. I was able to get my factory amp to work with the CD changer I installed so I did not have to install a bypass cable.