I've had a couple W123s where the firewall, just below the hood-hinge drains, rusted through behind the rubber undercoat, resulting in water in the rear footwells.
Tough to fix. Requires going inside the front wheelwell, peeling off all the loose undercoat, trimming & cleaning up as much rusted metal as possible, then patching & resealing. Since doing this to my 300TD last summmr, I haven't found any more wetness on the rear floor.
Also seen them rust-though the firewall behind the battery tray, but that's an easier fix.

Happy Motoring, Mark

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DrDKW