A chronic problem with many 122s, including wagons, is that the windshield
seal leaks and water gets into the fiber matting under the rubber floormat
and ferments. The resulting acid solution corrodes the floor through from
the inside. Not too long after I shipped my 122 wagon to Panama I drove
through a rather deep puddle and in the midst of the passage noted that
the passenger side floormat lifted up a couple inches and water came pouring
in. There was a hole big enough to drop a softball through. I have since
replaced all the floors in my wagon with 16 gauge sheet arn, finding that
gas welding with bifocals is a LOT different!

So pull up the floormats and check the metal underneath, not just visually
(because sometimes the rust is sort of laminated and there may be no solid
metal under it). If it is damp under there that will be a clew.
Also look at the floors (and the rest of the car) from underneath. You can
see a lot under there. Bubbles and humps in the undercoat are danger signs.

--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma