There should be no real problem installing the third seat. The seat consists of a backrest about 3" wide and upholstered and a seat of a similar type. These fold down into the opening in the rear. The seat faces the rear window.

I have a 1985 TD wagon with a rear seat, and it looks to me like this would be just a matter of getting the seat and backrest and perhaps some hardware.

It's not a seat for long trips: sort on like the back seat in a sports car: for children and short trips.

__________________
Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)