HELP!

i have one leak in my ERST which is quite fortunate but its really REALLY anoying me now, its in the rear quarter at the back where the sunroof pipe goes down! now here is a list of things ive checked:

The sunroof pipe with both air and water no leak!
The rear lights again all nicely sealed in and not leaking!
Under the passenger window (incase it was rotten under the seal), again no leak.

it has has a rear arch on it at some point so im more inclined to think it may be related to that but from the ammount of water that gets in there and i mean it can literly get to the top i must have a very big leak! can anybody think of anywhere else it may possibly be?
also it doesnt seem to fill up when being cleaned with a jet washer only seems to fill up whilst driving in the rain?!

 

You tried round the hinges for the tailgate at all. They're prone for rusting and letting water in..

 

ohh good thought! not had that one yet will inspect them 2morow! thanks