Active is the way to go to take up less space, the spare wheel well one i'm not sure about, as it sometimes helps the bass depending on where you position your sub in the boot, and you won't be able to move the spare wheel one.
Active is good, as there are no wires trailing around, or seperate amps to bump or anything , if you want to take the active sub out of the boot to save space, you just unplug it, whereas if you had a passive one , you will have speaker leads containing amplified current coming off the amp to contend with.
Not sure about the w208 clk structure ( we have a 430 but i have never taken it apart ) , i know on our w124 the petrol tank is behind the rear seat and it is a nightmare to get bass out of the boot and into the cabin..... being that yours is a cabrio. i would expect there to be reinforcing behind the rear seat which may pose a problem too ... best thing is to take the trim off in the boot and have a peek....
