Vitor has you on the right path with the door panel removal.
You will notice that the speakers are integrated into a complex plastic mounting.You will need to remove the whole assembly, then carefully ( and permanently) remove the speaker from the mounting-- The speaker is glued in! Remove any visible screws or push-in plstic rivets, then
take a sharp, sturdy blade and run it around the inside of the exterior rim of the mounting, then in the area on the side where the speaker can be accessed where it lies on the mounting, wiggle the blade in slowly to start the gap. Preserve the opening as you progress around the rim of the speaker ( I used various lengths of coathanger with flats ground on the ends). Eventually, Yoou can free the speaker from the plastic mount. There's a much better explanation, with pictures, soomewhere--I just can't locate it at the moment.
For replacement, either 6 1/2" or 6 3/4" can be made to fit. I used Polks in the rear and MB Quart in the front, but for the money, I'd use all Polks next time. (I'm running two 4-channel amps, so all 8 speakers are 4-ohm. Easy to get speakers, but the wiring was a bitch)
Main thing to do before you replace speakers is check your wiring diagram and see if you have 4-ohm speakers on 4-ohm channels by themselves, or pairs of 8-ohm speakers oh 4-ohm channels. If I recall correctly, the frontdoors of my '99 were 4-ohm, and there rear doors and rear deck were 8-ohm in parallel.
Depending on how you're wired, you may want to go with component speakers--tweeters in the dash, and mid/bass in the doors; otherwise, a coax in the doors will be fine. Post anyting you do that doesn't seem to be otherwise adequately covered. Good luck.
spend some time searching the brickboard; there are a lot of answers there.
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We have met the enemy and they is us. [Pogo] '99 S70 cop car : Rough Rider tires& suspension, Walmart fog lights, speakers by ebay, ambiance by Pall Mall, trim by Le Duc d'Tape