Did you have them check the sunroof??It could easily leak and land water anywhere in your car. Have them check the sunroof, make sure it fully closes in line with the roof of the car. My sunroof would always close and be slightly inward, it turns out that it was sticking partially,and the water wold seep into my rear carper.
1987 mercedes 300E
1995 e320 conversion(hated the 300e grill)
Monochromatic Paint (Custom Blue)
Flat Badged (front)
custom "carbon fiber" console
18 inch HP EVO rims
Sold! Now I drive a Monte Carlo SS