LOL - it's been a long time since we've had this discussion but... there are [what Porsche calls] speakers, more definitively [again according to Porsche] woofers in tuned ports located behind there. Usually a post like yours starts w/ "Help, my door speakers aren't working".
FWIW, their signal is from combining the front and rear channels below "X" crossover point.
Anyhow, it's easy to upgrade them to 6.75", or even 6x9" drivers. However, if you don't also swap out the amp, you'll be limited still to the lower range as the crossover is built into the amp. R&R is easy enough. Here are links for the BoxsterWiki to help you...
Comprehensive guide to upgrading the Boxster's speakers
Door Speaker Baffle Template
Good luck
