When I bought my 1998 S70 recently, among the problems were two blown rear speakers. Didn't want to replace with OE speakers, because they 1) are always inferior, 2) are over-priced and 3) can blow again. I went with Crutchfield's recommendation of Pioneer TS-A1780R 6.75 inch 4-way units. I bought them elsewhere, trusting that speaker fit would be obvious. Won't debate the merits of speaker brands, but I had previous good luck (and sound quality) with Pioneers.

Thanks to "Bryan Goldberg" and "Chris J" for their early postings on spreaker removal. It was not obvious, but simple if you follow their instructions.

To install the TS-A1780R, remove the 4 torx screws that hold the old speakers in the plastic frames. Shave 1/32 inch off of each new speaker tab (simple with a file or dremel) and the speaker drops to the bottom of the plastic frame like it was made for it. I attached the tabs to the frames with 3M double-sided foam-core tape. Bolt the frames back in the car. Splice the wires black-strip to black-strip, and replace the rear shelf cover. Half-hour job; sounds great!