Leaking REar Deck Lid

The rear deck lid on my Gen 4 4Runner SR5 is leaking. Water collects in the bottom of the deck lid and when I open the lid it pours out on to the carpet! When I open the lid and rock it up and down, I hear water sloshing inside the lid. I have checked the 3rd brake light and it seems to be sealed. Can water be leaking in around the rear window? If so, is replacing the seals around the glass the solution? Other possibilities and related fixes???

 

If you have a rear spoiler, check the washers. I've had to replace one or two in my time. My leak was not as bad though, but i would definitely check it out.

Cheers,

Sean