Do you access these through the panels at the end of the dash or does the little grille lever up

Levers up, gently

VERY gently otherwise you'll knacker the surrounding trim. I recomend a trim lifting tool, kind of like a screwdriver with the end bent a little and a V shape in it. Works a treat

or just bang it out from behind.

As ^^^ says, but do it on a warm day or the retention tabs will shatter. Or get the Hairdryer out

I just saw the extensive guide for tweeter swap in the Guide-section.
I could'nt reply there as it's locked, but here's another - simpler - way of doing it, whilst retaining the factory look (and keeping a few more thiefs out):

http://sfk.ibk.se/depa/showfile.php?carpic_no=2170&type=picture

You simply glue it on, I used a hotgluegun. Done!
Huge improvement of the sound + stereo ambience.

Cheers / Per