Ha ha ...you know what you're doing! I was going to mention 'conformal' ...but wanted to keep language plain. You're probably right, the water/moisture on the connector pins, or solder joints likely made things go wonky (for lack of a better word). At least part of the time. I figured just be design, there'd be moisture build up in the circuit board cavity if for no other reason than the board would heat the small metal cavity up, and it would cool down, (hundreds or thousands of times). Eventually, if not 'perfectly' sealed (and they all can't be 'perfectly' sealed), condensation would occur inside some trucks' racks ...even if water didn't outright leak in. It's really not a failure of the rack in most cases but a bad design that creates a secondary failure point.
Did you get rid of the rack already? It would be great to see what the motor area looked like.
Thanks for posting the pics. It was really enlighting.