I have a 1986 300 sl (euro) model. As you may know, the speaker selection is quite difficult for this model. I was determined not to "customize" my SL by drilling any holes to the doors or to the firewall (where the backseats would go) I replaced my front 4" speakers with Polk Audio ones. Anything is better than the OEM speakers. Though I was not able to add any real speakers in the rear, I wanted more bass from the system. The solution was to add Bazooka bass speakers. These basically fit in between the front seats and the floor board from the backseat. Note that you will lose about 6 inches of front seat adjustment.

I use more treble up front and add bass through the Bazookas. It's not perfect, but the sound is quite clean once it is balnced out.

Hope this helps.