I had this problem on a 123 and greasing the slides (well, just greasing everything that moved) fixed it. But I think I just got lucky.

I think a previous owner had this trouble and solved it by just standing down on the switch to make the roof force close. This would mean that the end of your cable very likely is eaten up.

But as tough as these are to find I would pull your cable out and make sure it needs replacing before buying another one.

You might also look for a cable from a 111. There is a good chance they are the same. The motors in the trunk are.