Nimrod

The tweeters can be changed without modifications if the new ones are small enough (I believe it was around 42 mm). If you install a aftermarket component set you will need an adapter for the speaker (Contact DeLander from diymobileaudio forum or buy from amazon but those usually are a litlle flimsy) and (easiest solution) also an adapter for the speaker cable (found on amazon). You should also dampen your doors and door cover, check e.g. the diymobileaudio forum for guidance on this, this is a very important step if you want good sound without rattling and other noises.

For me, disconnecting the center speaker helped a lot, made the sound a lot less "muddy mishmash".