The comments I make are about my 84 500SEL but I think your car is basically
the same. Under the rear seat cushion there is a motor with a drive cable on each end. One cable for the left side and one for the right side. On my car, one of the cables had slipped out on one side. I just pushed it back in and it has worked every since.
Has the car been sitting for a long time (old gas)? If so, As long as it runs try to use up the gas in it and fill it with premium gas, then take it out on the interstate and run it hard. These cars don't like old gas and slow driving.
The broken part in the floor is a foot rest. It is part of the left kick panel. I have never had one out, but I don't believe it would be too hard to replace.

Paul

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84 500 SEL (307,xxx miles)