I had the same problem with my 1990 300SL. I couldn't close the opened soft top anymore and the car only fastened the hard top partly. The back of the hard top was fastened as should, but the front fixings were just not tight enough (I could hardly see any difference with the normally "closed" hard top, but the hard top did just not fit the fixings until the end).
As long as the hard or soft top is not entirely closed, the switch will stay illuminated.
I had to bring my car to the MB dealer to get a new hydraulic valve and all was well again...
If you try to release the hard top, you should try it whilst the ignition key is turned on and the engine is not running. Flick the switch backwards (not forwards) and hold it there for a longer time to give the system some time to get some oil pressure...(it helped me getting off the not tightened hard top).
with a minor 600+k