1998 3/4 ton suburban with with 350 and 180k miles.

Car starts, runs and drives great EXCEPT

When left out in a rain it won't start. Drives fine in the rain ,just won't start after being left out [or goes through a car wash.] If I leave it running in a car was hno problems. If it is garaged no problems

What would be the starting place to look for where moisture is getting into the electric system?

If it were a 69 car I would say it is getting moisture in the distributor