Originally Posted by chylld
well first let's clear up what everything's called. a set of splits (more commonly referred to as just "splits" or "components") typically consists of a woofer (~6.5" diameter speaker), a tweeter (~1" diameter speaker) and a crossover. the crossover takes the signal from the HU/amplifier and sends high-frequency music to the tweeter, and low-mid-frequency music to the woofer.
also in the 06 civic, there is no provision for the tweeters in the a-pillars; the tweeter receptacle is located at the corners of the dash (the horizontal bit, not the vertical bit).
to answer your questions: in the australian version, yes there is nothing in the tweeter receptacles on the dash. so you can put tweeters there if you wish (not splits, as that would imply putting a tweeter, a woofer and a crossover in that tiny little space!)
you can change both front and rear if you want, however for sound quality reasons it is much better to concentrate on the front only (some pro sq competitors delete the rear speakers completely - after all when was the last time you went to a concert with the band behind you??)
the fronts would benefit most from upgrading the stock woofer to a splits setup, e.g. with the new woofer in the place of the old one, the tweeter in the stock location on the dash and the crossover somewhere where it won't get wet.
the rears (if you really must upgrade them) can be either upgraded woofers, coaxials or splits/components.
i would highly advise against getting a set of splits without a crossover - these are typically budget systems that use simple methods like inline capacitors to filter lower frequencies from the tweeters, and will add colouration to the music as well as limit the tuneability.