Make sure your sunroof is sliding freely before you repair the gearbox, or else it will strip again. It is a good time now to clean and lubricate the rails on the sunroof, and probably pull the cable out of the liner and clean the dried/dirty grease and replace it with new. I need to do this on my 83 300D, and just haven't had time yet. I bought a used sunroof motor assembly on eBay for $52 shipped from one of the yahoos selling a bunch of junkyard MB parts.