I made sure to find an 850 wagon with a third seat, and bought it. It's been used three times in four years. I believe they are reasonably safe, but:

a) instructions say not to use a child seat on the 3rd seat. I don't really see why, but there it is. So you can't have small kids sitting there.

b) there are no guides for the shoulder belts, which run really high so the belt cuts across the neck. (and you are not supposed to use a booster, which would have a belt guide)

c) not workable for older kids - not enough room for anyone >5 feet.

So you end up with a narrow range for the height of a person that can safely and comfortably sit there.

Plus, you loose all your cargo space. As for the wife, there is way too much %$#@$% in the back of our wagon to even think about flipping the seat up.

So - IF the kids are average height 7-12 years olds AND you are a neat person with an empty cargo space, it will work great.

My 5 cents.

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'97 850 n/a Wagon, 145K mi