Reviving a dead thread as I just had to deal with this. I also have this in my '96 and it is essentially a marketing gimmick. I just replaced my cheapo dual rear speakers with proper replacements to take advantage of the "8 speaker" system. I bought my speakers from crutchfield and the ONLY model they sell that works with this system is the Retrosound R69-N. They are dual voice coil speakers which is how they justify calling the 4 physical speakers an 8 speaker system. You can use regular 6x9 speakers, but since each set of wires is either the treble or bass, you can tell that something is off. It's expensive but my stereo sounds much better now. I would like to add that from what I and the people at Crutchfield could tell, the front speakers are just regular 5 1/4th speakers.
In case anybody needs help with wiring:
Left (Driver's) side:
Blue + Green -
Brown + Yellow -
Dark Blue + Light Blue -
Green + Grey -
These speakers obviously don't come with plugs to hook them up to the stock system, so make sure you wire them up to the correct voice. I do not know how to wire an amp up to these, but I've been considering installing one. If I do, I'll post an update.