There's a whole other thread on this somewhere. Probably in problems and fixes I think.
It has to do with how much water and salt you track into your car. Especially if you have carpeted mats vs all weather. It soaks in and seeps up to where your unit is. Causes rust and corrosion over time.
Likelihood probably goes snowy>rainy>dry for the issue. Vancouver is pretty rainy, but you probably don't carry much of it into the car.