Most of them have been listed but:
Sunroof drainage channels - start from the opening of the sunroof and make sure it's not blocked, go right down to the rubber grommet type exits either side of the boot floor. They have a little flap that can actually hinder rather than help. I snipped them both off years ago. make sure the tubes are pushed right into the rubber part (fnar fnar etc)
Rear windscreen wash - make sure the pipe is not loose or come off completely.
Rear windscreen seal, again, check for leaks particularly in the corners, can happen over time.
Rear windscreen lower trim - as mentioned check the little 'gaskets' and overall fit.
Boot weatherstrip - obviously check there is no damage, but check that it is seated properly too, many miss this. BEFORE you do that though. take it off and check for corrosion underneath, that can start to push the seal away. They usually suffer at the bottom,. in the middle where the join is.
Rear light seals - they have a gasket type seal that perishes over time. I had a POND in my car, I used to brake and it would spill over the rear seats, but because I so rarely use them I didn't notice for a while. THIS was the culprit on my car and, along with the seal itself not being put back firmly all round is my guess for most peoples problems
Boot alignment - you can raise/lower the boot with the wind in/out rubber boots, check you're getting a good seal overall.
Check for loose or missing grommets in the boot floor while you have it stripped out to dry.
HTH
