If it is the seals, you could try dusting some talc around the seals and then turning a hose on the hatch area. May indicate where it is leaking in.
Also, it could be coming up from a wet road, it may not be coming in from the top!
Otherwise I would recommend keeping the area dry and the hatch open (if poss-think security!) to stop it smelling until the dealer can have a look.
Steve - 2002 1.4SE petrol, Silver, black/black interior, Then an identical replacement 2003 A2. Now a Toyota Corolla 1.6 T-Spirit