Have a look at the parrot mki9200. Im looking to get one for the z4 if I can work out which dam cable it needs.
Speak to Phil at justcarkits, he'll point you in the right direction.
I did just this - having a big double din touch screen in the middle of the Saab dash sort of ruins the big wall'o buttons aesthetic. It's also a big PITA because of the various amps. The justcarkits lead was expensive (about £80

The Parrot kit works really nicely, I was torn between the mki9200 and the asteroid mini which is about the same price. I went for the mki9200 because the asteroid is meant to be slow to boot and is android based so could be a bit buggier than the low level firmware in the mki. Only issue I've got is that it mutes the radio when streaming audio, which is good. But when I'm not streaming it randomly cuts in for a few seconds then stops again - happens when I get an alert on the phone (which I understand) but also happens at random seemingly. I end up just turning off the Bluetooth on my phone when I'm listening to the radio but it's a bit annoying. It's still a good solution for a neat install and not expensive