My mother had a 86 300SL with a child seat in the rear...it's a nice feature but it can only be used in combination with very small children or older children without legs. There is just no room in the SL to let someone sit in the rear.
We took out the child seat and put in the normal carpented panel.
Adding the child seat is as simple as can be. We still have the black leather child seat lying on the attick (don't have the car anymore).

My 90 300SL has a child seat as well, but it is just of no use.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)