My car exhibited the very same symptom as satern's. Problem was the switch that detects the top is lowered into the compartment was stuck in the closed position.
Correcting that helped. But it didn't work 100% until I removed the control unit, partially disassembled it, and reinstalled. I think this final fix had nothing to do with taking the control unit apart and everthing to do with removing power to the unit, as Gilly explained.
You might consider checking for fault codes, as explained here:
Questions for techs on my SL, please read>>
and perorming diagnostics, if necessary, as explained here:
500sl Soft Top And Roll Bar