Water in trunk...

I am getting a lot of moisture in my trunk from time to time. I don't know why. The weather stripping arount the top of the trunk is all in good condition and seems intact. I lifted up the lining around most of the wheel wells and they seem imaculate with no rust.
Any ideas on how water is getting in?

1) Open truck, and clean your truck gutter.
2) Leave truck open.
3) Use a garden hose with a little water pressure, and let it roll on the rear window.
4) Watch the water roll from the rear window, down the trunk gutter, and roll behind your tail lights.
5) Use a black silicone sealant from Lowe's, and apply on outer edge of both tail lights,