It is located under the passenger rear speaker in the quarter panel and is secured with three bolts. Without a doubt the speakers are the weakest link with these systems. Most of the Probes came with the premium systems. Unless your car is a 251A without the Group 3 option, then it is a premium system. If it is a 251A without the Group 3 option, then it has to have the premium system as an option on the window sticker.
All models equipped with an equalizer are premium systems.
But this would be an exception rather than the rule. You can upgrade the system piece by piece if you so wish. A variety of low cost adapters are available to let you pick and choose the components you would like to replace. Ninety five percent of the 6 inch category speakers will fit with no problem. The rear speakers are 6x8 inch.