Hi Alex, my understanding is that Glycerine may help in softening the rubber bellows, but I would imagine that after a lengthy period measured in years it would be difficult to do much with the rubber bellows. Generally speaking, there are two problems associated with those bellows, the first being calcification of the alloy parts used in the units, and cracking of the rubber where it is close to the alloy metal ring that the rubber appears to me moulded to.
Generally speaking, it is the top bellows that are affected by the corrosion - I am pretty sure it's the tops that collect all the moisture and dirt that gets past the side window seals. The idea is to get as many units as possible from other cars, dismantle, rotary wire brush after taking the units apart. It is also important to wire brush the 'retainer hooks' that are attached to the alloy metal part that the rubber bellows are moulded to. It can all be a delicate operation and if you have enough dismantled spare bellows, you should be able to cobble together enough to get you out of trouble. There is always something satisfying when one can 'repair' quite simple items and make them work instead of having to outlay considerable amounts for complete replacement units. Try MB - they are surprisingly and nastily expensive. Regards Styria
N.B. One also needs to remember that the fuel filler flap and boot locking units are different to the units located inside the doors.