For a cargo of cars condensation is a second order problem. For example if the holds are clean the run-off doesn’t cause any damage.
The first time we had problems was from these deck vents:
Not the big blowers next to the rails but the so-called "skylight just to port of the fire main running down the center line. Sticking up above the deck are they cool down faster then the rest of the holds.
Over the ;years, a few dust storms etc this area got overlooked during hold cleaning. This was our first time getting dinged in this way.
Excess water was never a problem, the water led to other problems.