Vehicle is a 1983 300CD
Symptom
Both door checks inoperative. Doors uncontrolled.

Investigation
Removed door checks. Left side ball missing and spring broken. Right side strap physically broken.
Coupe door checks NLA. Checked EPC and found sedan REAR door check is same part number as coupe front door check. Still NLA.
Found aftermarket sedan rear door check at mercedes source dot com. These are an improved type in a plastic jacket to retard water intrusion. Bought two.

Repair
On arrival found that aftermarket door checks not a direct replacement. Modification to door check consisted of straightening the rear mounting tab. Also required enlarging rear mounting hole in door with a file so it would match with the door check mounting tab and enlarging front mounting holes in door slightly. Total time about two hours for the first side and one hour for the other since I knew what I was doing. Time includes R&R of inside door panel. Works as new now. Good to go for another 28 years.