For those interested Soundgate emailed me with the following:

"The interface you would need to install an after market head unit in the Saab is the XR4. This will allow the signal to match from the new radio to the factor amp with no damage, as long as it is installed correctly. The retail on this part is 99.95. Thanks CHET."

The XR4 sounds pretty interesting:

http://store.yahoo.com/logjam/soundoem4a.html