Water Leak

Opened rear hatch door on my 2005 Prius and noticed dampness on the floor mat on the left side. Looking closer, water is actually dripping out of the fabric around the cutout for the small light assembly in the rear area of the Prius. So somewhere in the rear door structure water is entering so it can run down to the light cutout where it then drips out onto the mat below. Driving in the rain seems to make no difference; it leaks if the car is just sitting in the rain.

Anyone experience anything similar? Any thoughts on where the moisture could be entering? I will be taking the Prius to the dealer, but it would sure be great if I could point out to them where the moisture enters so they could fix it correctly the first time.