My '86 300E has not given mr trouble from moisture in the distributor cap but several other cars of mine have done so. In all cases, the car would start, I'd drive a short distance, stop the car, and then have trouble restarting it. I believe that the moisture inside of the cap would evaporate from engine heat, then condense on the top of the cap near the spark wire terminals. This then shorted out the spark path.
I was able to eliminate this problem by smearing silicone grease around the edge of the distributor cap where it bolts to the engine. I also removed each spark wire on the cap and put silicone grease on the inside of the rubber boot to seal out moisture. I did the same at the coil as well.
I have found a big increase in electrical reliability by using that silicone grease on all electrical connections. In fact, there was a TSB in abour '93 or '94 from Volvo telling their dealers to do this whenever a car was brought in for any service. This is a good way to prevent problems with those fuses in the older MBs too. Same fuses in the older Volvos were a problem.
I but a large tube of 'SylGlide' brand silicone grease and use it on everything but my morning toast.
