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If the car has never been in an accident it is unlikely that the frame is bent. Unfortunately with any convertible it is difficult to keep large amounts of water from coming over the edge of the canvas and into the car with the window opened or the door opened. If everything is closed there should not be a leak. The weather strips on the quarter glass and the adjustments of the door glass are the key to keeping the car dry when the windows are up.
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With the door open and the front door glass rolled all the way up does the glass run flush with the rubber molding on the mirror or is there a step up or down when you get to the mirror? If the edge of the glass is not flush the glass is not adjusted properly. With all the windows up is there still leakage? There have been some issues with the alignment pins on the top where it latches. If there are dimples next to the holes for the alignment pins then the top is not latching straight. I am sure this would have been checked already but might be worth looking into.