IF you get a parts car, then the swap is easy. The donor will have all the parts, IF it is a factory original 3rd seat car. After market seats are available.
Which usually means no 2nd seat headrests.

The donor car will have seats, hardware, seat belts, side panels for the seat belts, carpet, under floor tray, and the headrests for the second seat. For the headrests you will have to remove and install the headrest receivers and MBtex grommets to the seats. Since the grommets are a slip click locking setup you may have to buy new units to install. The receivers are pop riveted in. Seat frames are ready to receive the units

All holes and mounts bolts are factory installed in all wagons.

I swapped my 3rd seat form another wagon I had to the current one. But I did not have the headrests ( they were not tranplanted, the seat was factory) . At the time of the swap, 1991, the headrest receiver kit was available from the dealer.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car