if its the hatch with a rust hole u can use double sided sticky tape for the time being like i have it really works
try looking around the tailgate and hatch those seem to be the only possibilites.
if you want to get it fixed try bringing it to a panel beater if its a " hole", but i use sticky tape and it works beautifully no water to be seen cause the previous was a fag and stuck a GT-wing on my car.
I am sure there is a hole somewhere, or possibibly the seals around the window hatch or tailgate just get water and start pouring to see where it starts to leak and kaboom thats your problem.
so panel beater, buy a replacement part of use sticky tape <--