Actually, you cannot depend on the Rear Seat Controls (RSC) or speakers to tell what Ford system an Explorer was originally equipped with. RSC's are NOT part of "Premium Sound", the base Cassette radio IS CAPABLE of interfacing with the RSC. The RSC option was SERIES dependent, not radio dependent (all XLT's, Eddie Bauers, and Limiteds have RSC's).
As for the speakers, well, there are three levels (Base, Premium, and Audiophile), same as the sound system levels. HOWEVER, at some time in 1997/1998, there was a cost-reduction action that eliminated the Audiophile two-way speaker enclosures in the rear doors, replacing them with plain "Premium" speakers (so, you might have an Audiophile system with Audiophile speakers in the front, and Premium speakers in the rear). At a later date, sometime in 2001/2002, a complexity reduction action resulted in even "Base" sound sytems getting "Premium" speakers.
The radio head unit, if it is the original one installed in the car, is the best way to tell what level the system is.
regards,
Jeff