I just renewed drivers seat in my 300SD and it now looks like it did in the showroom 20 years ago. It is a drastic improvement over the old tired seat that I had.
My spring base was broken in a couple of places so rather than paying big bucks for a new one I called Potomac and bought a spring frame from a passenger seat off of a later (after 89) model for $75. They sent me the entire bottom including a beautiful cover and the base including the motors. I specified a passenger side because I figured that it would have minimal wear...they re both the same. The horsehair pad was perfect so I did not have to get that either. I wasn't expecting the whole seat bottom but it is probably easier for them to throw the whole thing in a box and send it.
Cushions from the later models are much sturdier and even have "pool noodles" inside the coil springs. The new style seats also have clips that are easier to use when attaching the covers to the springs. If you are doing both seats try to get 2 new style passenger seat bottoms from a 90 or 91 car. The new style cushions and horsehair pads work fine with the old style leather seats. I think that the pads are different for MBtex.
Once I got the seat out of the car I was able to move the tracks back and forth to remove the power seat assembly from the spring frame by removing a ring and detaching the cable that goes from the motor to the gears that control forward and backward movement. It is just like a speedometer cable. I used the one from the extra power seat assembly attached to a drill to move the seat back and forth.
I bought the leather seat cover and armrest cover from ADSIT. (It was made by GAHH and was a perfect fit)
I went to a fabric store and got a dollars worth of black elastic to put along the top of my droopy seatback nets. I also got some foam to put under the seat along the front to get a perfect look. It took a couple of tries to get it to the point that it was taut everywhere.
Let me lknow if this answers your questions.

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL