Larry,
I do not know if that is an option you can buy in the U.S. However, I have seen plenty of late model SL in Europe actually have rear seats and belt assembly. They all look very OEM, and I know the European version actually have rear seat as an option. You may want to contact MBUSA to see if they can tell you more about it. One thing you may want to consider, it may not be legal in the US to install rear seats for the US version SL without going through some modification on the chassis. Rear seats were an option in earlier US models though.

95 R129
98 W163