Originally Posted by WANT '71 280SEL

I agree with fahgr, water in the cabin does usually come from the front. You need to also peel up the carpet in the RF so you can see the water come in. Water from a bad rear windshield seal is what usually ends up in the trunk wells rusting those out. Most likely your drains at the hinges are clogged. Water ends up in the back seat because it's lower thus it all runs there.
Thanks
David