Yes, subwoofer installation is extremely simple - just tap into wiring in the rear of the vehicle AND you can get power / ground in the trunk. Couldn't be easier, really. You may need some additional accessories depending on your amplifier and what type of inputs it can accept (and whether or not you have the required power / ground and fuse holder, etc.) Feel free to contact me if you're not sure what you have or need.
By the way, for those of you who are in the same position as Finndog, here's a link to the BSW Stage 1 Audio Upgrade for the X5:
http://www.bavariansoundwerks.com/pr...I=386/_cid=174
And for those considering a subwoofer solution, here's a link just for the heck of it!
http://www.bavariansoundwerks.com/pr...I=363/_cid=196