I need to do this to mine so I did look into it. The later cars had specific plates welded into the rear luggage area to which the belts were fixed. The rear carpets are different to accommodate this and the belt reels.
I have bought a set of rear belts to put in but have not got round to getting the plates welded in. I rang up
www.quickfitsbs.com in north london and they said they have fitted rear belts in a 944 a few times. I think they quoted £150 and it would take an hour. It was over a year ago.
Once my kids get a bit older and use seat bolsters rather than full on car seats I will be looking to do this.