if you are just replacing the speakers, you can cut the plug off the end of the factory wiring. and solder on the new connectors that come with your new speakers to the existing factory wiring.
that will work, and sound marginally better than the factory speakers. but it would mean that your cdplayer/head unit is powering the new speakers and you wont be getting the full potential out of them.
My reccomendation would be to get the wiring redone and power the new aftermarket speakers by an aftermarket amplifier. will be much louder and clearer.
if wiring up an amplifier, pulling off trims and re running speaker wire is not something your comfortable doing yourself, i would get a local car audio specialist\auto elec to do it for you.
hope this helps,
Timmo