Mazda MX5 2008 | Water Leak Found (part 3) Video

There are no surprises here, the door membranes are leaking. On a few cars the inner door membranes are moulded plastic panels, but on the vast majority of cars they are just a sheet of clear plastic held in place with a bead of heavy-duty sealant.

The windows have a rubber strip to keep out the majority of the water, but there is no way to waterproof these, so the door is designed to let any water that comes in around the window to drain out through the bottoms of the doors. The plastic membrane is supposed to keep all this water on the inside part of the door until it can drain out.

However, after about 7-10 years, the plastic sheet begins to dry out and shrink with age. As it shrinks, it wrinkles up and pulls away from the sealant and no longer does its job. These plastic sheets are cheap and primitive, but for the most part they do their job and cars have been made this way for decades. But plastic will perish over time, and how long they last really depends on what they have been exposed to in terms of temperature and humidity.

It's a fairly easy fix. We make our own door membranes out of a foam sheet, which is a little more durable.