I havn't taken the back seat out of an 850, but I'll bet that it is the same as a 740. The Haynes manual says - "Free the seat cushion from its retaining clips by lifting the front edge then remove the cushion."

About 6' back from the front of the seat bottom there are two bumps built into the floor of the car. Each bump has a slot cut into it, this slot is wider at its bottom. A large wire that goes across the seat bottom fits into these slots. Because the slot is wider at its bottom, the wire locks the seat bottom in place.

To remove the seat bottom you need to push it back about 1/2 an inch. If you push the seat bottom back too far, it catches on the back of the slot and won't come out.

The technique consists of pushing back and gently lifting at the same time. Release one side ar a time. Once both sides are released the seat bottom lifts right out.
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2 8s & 2 7s 600,000 miles total