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Check the wiring diagrams first, and compare them to the actual wiring. If you have pairs of 8-ohm speakers in parallel, a straight speaker-for-speaker swap with 4-ohm replacements could fry your rig. The wiring diagram --and searching this board with the model # of your head unit--will also help tell you whether you are running all the speakers off the headunit, or whether you have a separate amp running the additional speakers.

That said, unless you are a hyperpurist, even medium-priced speakers such as infinity or Polk will sound quite good with the factory headunits and amps. If you want to spend more, and have the ears to appreciate it, there are a lot of MB Quart speakers and other expensive goodies. Check Crutchfield's to get the general idea, then check ebay if you want to take some risk and do without a service dept or warranty.

Sizes will depend on what size holes are there and the available depth, and possibly some ingenuity in mounting. Advise how the project goes; what you learn may help the next guy avoid a problem.