86-89 should have the same connectors but you have to replace memory with memory and non memory with non memory.

It's not too difficult to transfer the seat cushion and seat back to a different track. It's not fun but not difficult. I have a set of seats out of a 92 300E in my 87 300D. The memory connectors from 90 are different so I transferred the 92 seat to my 87 memory track. I didn't have to change the non memory passenger side track.

Plug - the original light gray leather seats of my 87 300D are still available!

Sixto
87 300D