Water can get past window seals and inside the doors when it rains or when a car goes through a car wash, and the water is supposed to exit through drain holes at the bottom. Those holes in the bottom of the doors can get clogged by dirt and road debris, and when that happens water accumulates inside the door and causes rust. You can see signs of that on cars that have rust forming at the bottom of the doors, or rusty trails running across a rocker panel (the body panel below the doors).
Most vehicles also have drain holes in the rocker panels and rear quarter panels (the body section behind the doors) and other places that can clog, trap water and cause corrosion.
High-pressure car washes (or a carefully aimed hose in your driveway) can clean debris from some drain holes, but cars that seldom see the inside of a car wash or spend most of their lives in dirty environments can be prone to clogged drain holes. In addition, some vehicles fall victim to rust because they have too few drain holes or small ones that are easily clogged.
Checking these drain holes is not typically included in a professional hand car wash or vehicle detailing, but it is something an owner can ask to have looked at.
One solution is to get down on your hands and knees once a year or so and make sure the drain holes are clear. You can poke into them with a coat hanger, screwdriver or similar object to clean out any debris.
