I put 6x9s in the storage compartment area on my SDs back deck and a 12" sub and box in the trunk. The trunk is fine and you can port it into the passenger compartment if you desire. Ive got a writeup, I'll throw in a link when I get off my phone

Edit: here you go http://benzbeat.blogspot.com/2011/02/w126-300sd-custom-6x9-speaker-and-back.html

Easy to make, sounds fantastic, looks good and not gaudy. The 6x9s have enough bass by themselves to carry the low frequencies, but the addition of a 12" & ported box in the trunk completes it. I currently have the sub and amp in my 300E, the 6x9s with no HPF sound great paired with 4x6's in the front.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth