I finally did some upgrades on my wife's 2005 X3 this last weekend and found a couple of resources on youtube quite worthwhile for how to open up the door panels for speaker installs:
These instructions were a bit short for my wife's 2005 model as there are also screws at the back of the door opener assembly but everything else was a workable tutorial on how to take the door apart. I will leave aside the attitude of the "I am so good at this I forgot some details" dude narrating the video but some reading this thread might find the visuals on pulling the panels useful. I certainly did.
Another one for opening up the area around the radio:
In my case I did use the Bavarian Soundwerks speakers and added an Alpine KTP-445 inline amp to our existing Alpine iDA-X300 HU and pretty much smothered the backsides of the door panels with dynamat while I had it open. I already had replaced the under seat subwiffers (sic) with SWS-8's powered by a 300W Focal mono amp and the end result is far more passable for something I can listen to on the road compared to where we started from. Good solid bass response from an honest subwoofer would really help but my wife refuses to give up even an inch of space in the back when it comes to her cargo space.
AKA AlanL on all other forums
2005 X3 2.5i - Mystic Blue
2010 328ix Sports Wagon - Space Gray