Do a search for "vapor barrier"...its a very common malady on the e38/e39 forums and there's plenty of DIYs on the forums and even YouTube.

Most times, the adhesive that holds the vapor barrier to the door fails. Thus water that enters into the cavity of the door will escape where the adhesive on the vapor barrier has failed, instead of continuing on to the bottom of the door where it flows out the built in drainage slits, onto the rocker, then out to the ground.

When the vapor barrier fails, the water will flow over into the inside of the car (based on how the design of the door comes in over the top of the rocker...and the water leaks inside the vehicle instead of to the exterior.

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