Ok, I did a little checking. The part # is 979780 and it is what volvo calls a 6MM flange screw. (I'm getting deja vu now: I needed one of these when I went to install my pa-300 and I could not find anything at my local hardware store...) I don't remember if it is standard thread or not.
And, I am going to guess that you might be able to get one from a certified Volvo truck & parts dealer, too; yup that's t-r-u-c-k-s. Some fasteners (and I think this is one) are shared with Volvo trucks.
Well that stinks--I hope I didn't give you the wrong part #, I'll check again: yup, it says 979780 (M6). Now this is from VADIS 2004B, which obviously does not include MY '06, however, I think that part is the same.
The bolt (it looks more like a bolt than a screw) is listed in VIDA and your dealer parts guy should be able to look it up. I called a local dealer and they confirmed the part # ('06 is same part# as '04)--I even ordered a couple for myself, truth be told, I never bolted my pa-300 in when I installed it!
So what could be going on here?
-they gave you the wrong part? VADIS/VIDA lists the wrong part?
-we are not talking about the same thing (the two bolts that go into the bracket that is part of the amp and holds it down)?
-the threads on the nut are stripped?
-you are looking at your Saab or you went to the Saab dealer for the part?
Was there any packaging with the bolt? Got a part #? Was it too small by any chance?
I don't know if this helps, but when I installed the OEM amp in my 2001 V70, the captive nuts were there on the chassis and were all welded in the right place, but they had not been threaded. They just had a straight hole in them, and I needed to run an M6 tap through them.