To fit the memory seat to the drivers side, you'll need the memory seat wiring loom and switch. The loom runs from the switch in the door to the seat control module under the seat.
For the passengers side you'll need: switch, loom, switch covering, door pull handle covering ( and to cut the door card to accept the switch). The switches are slightly different between early and late cars iirc.
Are you sure the "new" passenger seat is memory?
They should but....
IIRC the pattern on the leather is different between early and later cars. You may find that the front seats don't match the rear seats.
Are you sure they are leather and not MB tex?
The passenger seat will be a straight swap, it depends on what you want to do on the drivers side. If you are happy to have manual adjustment, you can unplug the motor connections under the seat, remove the fuse for the seat ( fuse E on mine), and fit the manual seat. You should notice that the "new" seats have an extra connector for the occupancy sensor ( unless you have airbags, which were very rare in 1988), that you won't have.
If you are swapping seats because yours are worn, the seat covers can be swapped. The base is quite easy but the backrest is a bit more of a pain. There are various videos on the subject available available from a well known website. This would allow you to keep the electric functions of the drivers seat, but you will have to consider what to do about the "hole" left where the manual adjustment knob is on the manual seat cover. ( on the side of the seatback facing the passenger seat)
I found that the electric seat offers a slightly better range of adjustments but that may just be me!
Thanks for the info Andy. Not sure if the seats are mb tex or leather. The chap did say they were leather but i dont mind what type they are as they will be a vast improvement on my cream cloth seats. I dont mind manual adjustment. Less to go wrong.