The drains to the back of the car are plugged on a rubber hose nearly at the end of the roof just in front of the rear screen. If you take away the upholstery at the rear pillar and remove the rear right handle, it should nearly be possible to get to the hoses' connection.
The other end of the rubber hose goes out of the trunk behind the bumper (just behind the wheel/fender) It may be easier to check the hose from that side...

greetingz,

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