Maybe the sunroof is dripping some water? I think the rear edge of it is right over the back of the front seats. I'll ask the stupid question, does this only happen when it rains?

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C