I also have a 1990 SL. Softtop is very slow to move up/down when it is hot (over 25 C, 80 F) weather. Mechanic states that the motor that runs the hydraulic system for the roof needs replacing. Only $1800 USD!!!
In the meantime, I use the wrench that came in the toolkit to manually latch the roof when the red light doesn't go out. Refer to the owner's manual for instructions for manual roof latching. My problem is with the front latches. Swing sun visors out of the way, remove small plugs in the cross pillar, and insert wrench and turn one or two clicks. If the rear pins aren't latching either, you get at them from the trunk. Hope this helps.