OK, just did this job over the last three weekends on my 940.
If it is just the back one you cannot get off because you cannot get a clear shot with an impact (you can barely get a shallow socket on that one with a wobble!) you honestly are better taking off the the exhaust manifold assuming it is a non-turbo. If it is all three you cannot get of with a quality impact wrench, you MUST take off the exhaust manifold.
I got two of three out "easily" by grinding the tapper off a 9/16" shallow impact socket (my hardware was 15mm but yours could be 17mm). I had already soaked them for three days in penetrating oil AND took a wire wheel on a dremel to all the areas I could get to removing rust from the threads. I then put a long extension on the modified socket and pounded it on with a 3# hammer. The 9/16" is technically a little too small for a 15mm nut but rust had eaten away enough it went on after about 5 whacks. Then I hit it with forward 3 seconds, backward 3 seconds, rinse repeat until it all at once let loose in both cases.
The third one was not to be so I ended up pulling the manifold anyway. I did end up rounding off the nut enough I had to weld another nut to it but with it out of the car I could get 90% weld around the nut.
A nut splitter does not fit on that one by the way...in or out of the car. The infamous guru of the Brickboard, Art Benstein, maintains 11 of these cars and by his own admission, he does not even try to take them off in the car anymore and drills them after to replace with stainless hardware.