Basically, a 10mm, six point socket should do the trick. Those nuts should not have a whole lot of torque on them, so if it's really giving you trouble, spray some penetrating oil on it for a little while, and then go back at it with the socket, vise grips, or whatever you can fit in there that won't ruin it. A hardware store should have those bolts. They're very standard. Or you can look around and steal one from elsewhere in the car... one that isn't necessary.
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1992 940 wagon, low miles
as well as others.