Well after all the years I've spent looking at navigations and wanting to get one, I think I'm finally going to take the plunge and get the AVIC-D3 for my 91 Volvo 940SE.
Basically, there are 2 single DIN openings which when both are removed, make a double DIN opening. The stock radio fills one of these openings and the other is a storage space. My feeling is that with both of these removed it should allow for a double DIN unit like the AVIC-D3 to slide right in.
Here is a picture, though it doesn't do much justice, this is not from my vehicle as its in the body shop currently: