I know which one you're talking about - I broke one of mine too when I upgraded the door speakers. It'll stay on fine...mine's been for 4+ years. Anyhow, to get it off properly, you need to release the bottom part gently but don't take it all the way off. Then reach in w/ a skinny flathead screwdriver and push down to release the hidden tab underneath. Details are on page 6 of Andy_M's guide to upgrading the Boxster's sound system over on Renntech or the Boxster Wiki.
Here's a link to the thread w/ the downloadable pdf on Renntech or you can PM me w/ your email address and I'll send you a copy.
Good luck and looking forward to seeing your after pix
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