Had this problem in my 93 945. Volvo came to the party with what they call 'goodwill'. The car was 6 years old and had a dealer service history. Goodwill was 25% of the quoted repair cost. I ended up repairing it with the assistance of my neighbour.

The problem was rust in the seam at the lowest section of the window surround in the rear doors and to a lesser extent in a front door. This is the vertical seam closest to the central pillar. I posted a detailed warning on Brickboard. This corrosion can very easily go unnoticed as it is hidden by a rubber seal.

I have noticed the exact problem on several 740/940s.

Repair involved carefully prising the seam open with a right angle, inch wide tool that I fashioned out of some metal. Using a hair dryer to provide heat to the exposed exterior paint during the opening and closing helped to prevent the paint cracking. This was followed with the full Por 15 treatment. It was then resealed. Great care is needed not to harm the exterior body surface.

The point of entry for the moisture into the seam was not apparent.

Repair was carried out 2 years ago and seems to have been a success.

Take care,
John H from Downunder