Mine did the exact same thing as yours. It worked perfectly for years and then one day would only open half way. What I did was lube the rails on the part that would open. Then close and reopen several times but only as far as it will easily open (and close). Then open, clean, and lube again. Do those steps several times cleaning out the newly dirtied up lube each time. Keep working it open more and more each time and you'll have it. Also look back into the whole assembly and make sure nothing is blocking it.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230