My 1975 s2 , no sunroof has admittedly been out in the rain, i just noticed in front of drivers seat the underfelt and floor mat is soaked. No apparent reason for this what would typically be the source of water coming in? LH side is dry as a bone.

 

one way water soaks the floor is thru the a/c matrix drain tube that gets clogged. To unclog it, you need to raise the car on a lift and find the drain tubes on each side of the transmission pushing a straightened metal clothes hanger or similar up the tubes.

Another possible source are the cowl drains seen at each side of the firewall bulkhead, (engine side). The tubes have a "membrane" at the end of the rubber tube that can clog, allowing water to collect at the Cowl by the wipers.

 

I have the same model and had the same problem, main cause is the wiper are stems, the shroud sealing rubbers die, silicon is just as good

 

For me the problem was the seal under the brake booster bracket/pedal box. Poorly installed from the factory but the car was apparently garaged forever so only slight rust until I started restoring it and washing it regularly. Pretty easy fix with a new seal, you can make one yourself out of rubber sheet or choice of sealant.

 

The drain tubes under the plenum, or cowl , the rh one seemed to be partially blocked. No further issues after clearing it a bit