Removing rear seats in a 1990 S4:

There are 4 bolts that hold each seat back in place- two at each side and two on the side towards the centre. The centre console between the seats has to come out and there is a bit of juggling around if you have the rear ac fitted or whatever replaces it.
Fred R
There is a trick to getting allen wrench onto lower bolt of outboard bracket.

Put seatback into down position.
that is, flat like a cargo floor.

Next, release the seatback.
Instead of raising, over rotate so cargo floor tilts downward towards front seats.

That gives access to the bottom outboard allen bolts.
I know this works up to 86.5.

I think it is true for S4 too
that did the trick thanks, removed centre console and the other bolt appeared, seats out now. atb