Hi Nick,
Heater control panel:
I had the same issue on mine, and after asking around on here and Porsche Club, the best option was to buy new (not expensive in the scheme of things) - got mine from Max at Frazerpart, who is always a pleasant and helpful chap to deal with.
Rear drivers suspension sagging:
Again, I had the same issue on mine. Seems these cars suffer slightly from having a big hump of man sitting on their right hand side driving them for 20 odd years! Apparently torsion bar re-indexing can be a PITA job, if not doing it yourself can rack up a lot in labour hours, and sometime various bolts are horribly seized. I got scared and didn't attempt it myself, thats not to say that somebody much more competent on here will have the knowledge and experience to help you though. However, take a look at these, in case you haven't already:
http://web.archive.org/web/200412152007 ... &id=62&c=4AND
http://boerger.golden-tech.com/ride.htmThere are some eccentric bolts that allow about +/- 3/4 inch adjustment on the rear ride height for each side. I managed to get mine suitably level by playing with these - much easier!

I also changed the rear shocks whilst in there, as they were totally knackered! Probably worth bearing in mind as well that the front struts can be worn too, and in my case I need to sort the drivers side front eventually, as this is pulling the whole side of the car down slightly. More ££££s

Hope thats of some help...
James