My van will not start when it is wet out. otherwise it starts and runs terrific on 1994 Chevrolet Lumina APV

I only have a problem starting when its wet out and my van has sat more than 8 hours. I was told most likely a crack in the distributor cap.Any ideas or suggestions? Also any step by step instructions how to change cap and rotor as I;ve heard they should both be done.

Asked by for the 1994 Chevrolet Lumina APV
Does not have a distributor type ignition system. Suggest getting a mechanic to check it out! Very tough to access the ignition componets on this vehicle, even for a pro.
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Try spraying plug wires with wd40