i havnt had this problem with my celica (only the condensation buildup)
but my guess would be with the weatherstripping...i'd probably get it replaced before the next rain comes.
1.buy some baby powder
2.check weather channel
3.when rain is coming, apply the baby powder generously to eveyr area of your weatherstripping (think paint coat...except replace paint with powder)
4.open your hatch SLOWLY and AFTER the rain is DRIED up from your celica (don't open while it still wet...you most likely will drip water onto your weatherstripping just from the runoff)
5.check for areas in the weatherstripping where there is no powder (the water would have taken the powder with it on its way down to the spare area) this should also (unless you have a big hole in your stripping) leave a slight trail of dried up powder in a waterline fashion.
6.you discovered your problem area..check it out and see if you can fix it without replacing all of your weatherstripping.
if the weather stripping is NOT your problem..then either your hatch hasnt been closed all of the way or you have cracked glass (either of which i'm sure you would have already spotted--which is way i'm almost certain its your weather stripping.)