Yet another water ingress question

[MWMan]

I know there is probably a thread on the forum covering this but I have searched and cannot seem to find one that covers my problem. I opened the Tourer tail gate this morning, and was going to put something in the small recess on the left side of the car. Low and behold it is soaking wet. The right hand side is bone dry. I haven't had chance to have a good luck round yet as to where it might be coming from. Can anyone point me in the right direction?

thanks

John