In my W210, I'm currently using 1 12" JL Audio W3 sub in a simple sealed enclosure. In your rear deck, you have two 6" cutouts per side. One has a speaker, the other just foam. I just cut the foam out so the bass could port into the cabin. It probably wasn't necessary, but I wasn't sure what kind of effect the gas tank was going to have.

The JL is designed to work in small enclosures, but one disadvantage to this particular model is that it is deep. The minimum depth for my box was just under 9", including a trim panel so the sub would appear flush with the box. I made the box to have 2" "legs" on the ends so I could get extra clearance for the spare, as the box overlaps the spare by about 4.5".

Hope this helps

Lloyd