We replaced the speakers in our dash this summer. The driver's side was a bit tight so we loosened the bolts in that area to give my hands a bit more room to get in there. Reach up behind and unbolt the speakers. The nuts are stamped spring steel. One little hint from someone who's had their hand in a lot of door panels where you can't "see" what you're doing. Close your eyes, they're worthless anyway. Let your fingers "see" for you. If you feel lightly and close your eyes so you're not distracted by normal sight, you can visualize what your fingers are telling you.
As for the wiring, as said above, pull it back to the head unit, then reroute it to the dash speaker locations. When you pull the OEM speakers, check the wire color, the PO may have simply cut them at the head unit and you'll likely be able to just reconnect them to the head unit. Check your Chilton's for polarity.