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A door's drain holes clogged, so water that has entered the door cannot leave it.

The thing most people do not know is that a door is supposed to have water in it. Rain runs down the outside of the glass, past the weather strip, into the cavity — that is by design, and drain holes along the bottom edge let it out again. The system only fails when those holes block, which they do with leaf litter, road grit, pollen and old sealant, at which point the door fills like a bucket.

Everything inside is then submerged or sitting in permanent damp: the window regulator and its cables, the lock actuator, the speaker, and — the expensive part — the door module and its connectors. The corrosion that follows is slow, invisible from outside, and produces exactly the intermittent then permanent faults described under water ingress.

The signs are noticeable before the electronics fail, and worth acting on. A sloshing sound from the door over speed bumps, a persistently misted window, a musty smell, a speaker that has gone crackly or dead, or a damp carpet where the water eventually overflows inward past the vapour barrier.

Clearing the drains is cheap, quick and almost never done — the holes are along the bottom edge of the door, and freeing them with a soft probe takes minutes. It is the definition of preventive maintenance here, because the alternative is a door module, a regulator or a speaker, and a new part fitted into a door that still fills with water simply fails again.