325power -- You are in good shape, even without parcel shelf mods. The BassLink "servo control" is the main reason. For an application as a self-powered subwoofer intended to be mounted in the trunk, the servo-control is indispensable – it compensates for changes in the enclosure (i.e. your trunk) as the shape and volume of the enclosure changes (i.e. different sizes/shapes of trunk between 3-, 5- and 7-series Bimmers, and as you put things in or take them out of your trunk).
I put my BassLink in the trunk of my BMW (740) and my buddies have BassLink in their 330, 530 and 735! The stock audio system found on all BMWs are severely wanting in the low end. It turns out that the BMW amps (both standard and Premium/DSP) are *really good* in the midrange and highs; it is just the lows that is too wimpy. The BassLink fixes this, and fixes it “but good!”
There are several kits around to help with installation. For BMW, I recommend the BassLink Install kits you can find on eBay (just go to ebay.com and use the keyword search for “BassLink Install”). The kits will save you a lot of time and they have all the components you need – there is even one that has a ‘QuickRelease’ mechanical mount so you can take the BassLink out (and replace it) in under 60 seconds. The kit is highly recommended as (1) it has all the information you need (2) it will save you a lot of time, and (3) it has the hardware – including the “QuickRelease” mount.
There is a good article on Basslink for BMW at:
http://www.wakajawaka.org/bmw-audio-upgrade/
PermaGrin (740 plus awesome tunes will give any male 25-65 a PermaGrin)!