I've got an HK stereo and a similar rattle.. I localized mine to the interior panel on the passenger sides adjacent to the back seat. If I press lightly on the panel in front of the rear arm rest the rattle goes away. So I took it to the dealer for a fix.. they made the passenger side worse and "fixed" the driver side (which didn't rattle) without my approval and now I've got the rattle on that side too. Took it back again, another fix and they're both exactly the same. For now I've given up, but if someone finds a fix PLEASE share bc that rattle is the only niggle spoiling an otherwise perfect driving experience (mine's got 19,000 miles on it now bc I can't get enough).
I also had the squeak from the center armrest. I wd-40'd lightly between the hinges at the back of the armrest, moved it up and did the same from the front then moved it up/down to work it in. Because the gaps are so small it made a bit of a mess, but was easily cleaned and the it hasn't made a noise since (prob 6-8 months ago).