Mmmm, I think the only solution is to pull the plastic trim off (the big butty bit), fold the rear seats down, lie upside down in the rear so you are looking up at the hatch, then get someone to wash the rear hatch area with a hose as though it was raining, then try to see with a torch as to where it might be coming in. Shouldn't be too hard to trace.
I won't be posting any more about my hatch repairs as that particular Supra is no more ... long story .. but I do have a new Supra (well, new second hand) and from what I have seen it might not be leaking. Granted if it is I will be posting my findings here but that is a fair way off before I get that far.