There are a few things that can cause that besides the neck, just so you know.
If your car has the trim from a vinyl top, check the attaching points for it in the trunk as they can leak where they come through.
Of course there is the rear window seal.
Then there is also the channel under the window chrome trim that is basically a trough an inch deep that catches dirt and water and holds it there until it gets a chance to dry out.
They can rust in that channel really easy, and that leaks into the trunk.
You pretty much have to get in the trunk with a good light and check it out, and then there is always the option of getting in the trunk and closing it, then have someone spray the car while you watch for it to come in.
