You can blast the area with a hose all you want and that will seldom disclose a leak. One test that you can try is to soap down the outside of the window frame and then have someone on the inside blow pressurized air along the frame with an air compressor. You should see bubbles where there is a leak.
If it's coming from above then you have a roof leak.
I'm confused. You started off this post saying that you had a window leak and then went on to say you had the dealer try to fix it. If you told them it was a window leak too, maybe that's why they couldn't fix it?