Finished up the install on the subwoofer in the trunk of my son's 2008 Cooper S. Found wiring for the H/K system here on the forum that matched my plug. (Search "R56 HiFi wiring connector diagram")

Used a Rockville RW10CA a bit overkill but this was cheaper than the 8" ones that I could find. I started by tapping the rear speakers and using the High Level inputs, but the amp/sub kept going into overload because I was clipping the allowable level on the inputs. Moved my taps to the rear speaker inputs to the amp and it was better but still ran into the clipping from time to time.

Went with a Line out converter with adjustable gain and ran to the low level inputs of the sub with the addition of the amp turn on wire from the H/K amp. No clipping, no overloading. Sub has a remote gain adjust that is mounted in the console to control the sub from driver's seat.
Line Out converter was put in the left pocket behind the H/K amp.

Box was made with plywood and 3/4 boards. Painted the base black and the top cover with truck bed paint for durability. Mounted a 10" grill over the sub and left a small section to keep the quart of oil and other miscellaneous items. Did not plan well for the cover as it hits the side panels and doesn't allow the lid to open all the way. Not much in there any way!

 

I like it looks pretty good.

 

schweeet!