Search this site for info--there's a note on removal of the rear deck cover to get at the speakers. If your '98 is like my '99, you fold down the rear seat backs, remove the side seat bolsters, then remove the plastic trim from the rear post and rear quarter window; then lift up the front edge of the rear deck cover to free it from the clips, and slide it forward. Sounds harder than it is, at least the second time.
Check the wiring diagrams! if your rear deck speakers are in parallel with the rear door speakers, as on the '99, you will have 8-ohm speakers which are a bit harder to find than the industry-standard 4-ohm aftermarket speakers. If you repplace the rear deck with 4-ohm speakers and do not do anything else with the wirng, you may fry the circuit. One possible way around it is to switch to all 4-ohm speakers on a 3-way crossover, controlling 8" bass in the rear deck, 6 3/4" mids in the doors, and tweeters wherever you like them in the rear. Otherwise, if you go to 4-ohm, you will need to either add another pair of channels somehow and rewire so that the rear door is not in parallel with the rear deck, or add an appropriate resistor in series with each of the rear deck speakers to bring them up to 8 ohms if you are going to keep the present wiring and rear door speakers.
please post any results so the next guy can learn from you.
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We have met the enemy and they is us. Pogo '99 S70 cop car : T Roosevelt Rough Rider tires& suspension, Walmart fog/driving lights, speakers by ebay, 2 old Volvo amps cranking out 8x20 W, ambiance by Pall Mall, trim by Le Duc d'Tape