I had a similar problem when I 1st got my car last June. I didn't have leakage into the car but my weather stripping or cowl trim under the windshield area and around the interior of the hood was severly worn out. The BMW dealer in NYC replaced and resealed the area for around $120.
The general theory behind this is when you take your car to the carwash or if you use harsh chemicals when washing it slowly eats away at the rubber and vinyl 'till it starts to dry and crack. Also persistant exposure to acidic rain and road salt can create similar situations
Apart from this you could also have other things wrong thats causing your leaks. I'm also not too sure about the convertable versions.
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