Water in trunk after heavy rain

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 investigating for colour (rust) and have found surface rust on the underneath side of the back windshield deck (obviously from water trickling/collecting there from wherever the source may be). I have seen enough pictures of Valiants and Darts rusted out where the roof meets the rear quarter panel to think that may be the entry point. I have yet to remove the interior plastic/fiber panel (located between the rear door window frame and the rear windshield glass) to investigate as to the extent of rust damage, if that is indeed where the water is coming from. I'm thinking that there is water coming from underneath the vinyl top where the drip rails sit and running down to collect and pool and rust and make its way into the trunk. Mine leaks sitting in the rain and when driving.

I've had a look at the windshield seals and the trunk seals and not found them to be the problem... yet. The water appears to be coming from up higher than the windshield seal. There's wet up above the wheelhouse/tub between the inside of the quarter panel/roof weld and the near side structure (rear deck) of the car.

Slant6/4Speed mentioned the possibility of water splashing from the wheel wells when driving and I thought this to be the case as well, but the water is very definitely coming from the same place whether it is driven or if it sits for any amount of time. From up (several inches) above the windshield seal.

and, Patrick's74Custom, were you successful in fixing your leak?