Have the same problem with my '69 230/8 only on the right side. Had lots of rain lately here in Fla. My local mechanic says it could be the drains for the vent in front of the windshield, firewall seals or the antenna seal. When the water comes down there it sloshes back under the seats when you get under way. Also it could be seeping down the windows or, for W114/115s, through the rear window seal. Then you' d get it in the trunk, too. Good luck.