I had that happen to my 1991 300SL as well. I couldn't figure out why it happened, it just did. Perhaps the lubrication dried out, as there was no sign of grease in the fittings. In any case, I replaced the ball ended stud as well as the cap and it's been working fine for years. I do believe that it's a good idea to use some white grease on any of the joints which move or flex and I try to do that every other summer. This car ( 90 -93 ) has the notoriously awful soft top control computer. Much has been written on this forum on how to read out the trouble codes and reset the controller when it hangs up, which it eventually will. You can build a home built tester to read and reset the codes. It's not difficult, but it can be a PITA. Use the search function to find the info on the controller. The soft top controller interacts with the roll bar controller and the power windows. Fuses for these are located in the trunk, upper right side. Be sure to check the hydraulic fluid level of the system under the spare tire. BTW, later year models of the SL had an improved STC.

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