Yeah.... that sounds like pretty much the same thing I'm going to have to do as well. Still haven't positively found the source of the leak, but apart from tearing out the whole back seat and rummaging around upside down in there I don't know what to do.
I am fortunate to have covered...
My '75 4 dr Valiant trunk has water leaks too! The water appears to be coming from somewhere just above where the wheel tubs are located (water drips down the tubs and pools in the low spots of the trunk pan behind the spare tire well). I have crawled around inside the trunk with a flashlight...