Does anybody happen to know if electronic seats from a 1992 400E fit/work in a 1987 300D? What about the rear seats, are they interchangeable? My originals seats have seen better days.

Both front and rear will physically fit.

But, I do not know about the match up of the 87 electric seat control circuit to the 92 set up. I know there is a difference between the memory and non-memory circuits.

there is a difference between memory and non-memory, the complete wiring harness on the body end is different. but the bolt pattern of the seat holes is the same.

if you're doing the fronts, be sure to use the 400e seats, with the sliders/mechanism of the 300d.