Previously I had installed new front speakers and new rear speakers in the car. While I had the interior out for other jobs, I replaced the factory subwoofer and amp. The Kicker sub did fit into the stock enclosure, but it required a wood adapter ring in order to get the depth I needed.
On the interior, when you have the car apart, every tab that is broken should be repaired or replaced. Each tab and each screw needs to be there or you will get rattles. There are no shortcuts, this is the Bavarian way. Previous owner(s) of this car had broken various tabs and there were missing fasteners. I replaced and repaired everything I found. In addition, I added felt tape to any plastic-on-plastic connections that seemed to squeak. I wrapped the extra amplifier wiring in felt too, just in case it would rub up against something and cause extra noise.
Alpine Amplifier PDR V75, $499.00r
Alpine Subwoofer remote level controller, $29.99r
Kicker 40CWRT672 Subwoofer, $119.99r
Miscellaneous Connectors, power cables, RCA patch cords, $54.85 from Crutchfield
Trim, black, left side, 51438400117, $31.89r
Trim, black, right side, 51438400118, $31.89r
Total cost of this mod = $767.61r