W123 ones are quite expensive, though I find these cars in the junkyard often enough now. If my housing failed (OP's appears cracked), I would just run 5/8" heater hose (I use silicone, lasts forever & more flexible) where the monovalve sat, then install a later one from a W124 (cheaper, I have one from ebay) where the aux water pump (electric) sits (front passenger side). The aux pump isn't needed (perhaps in Germany or Alaska) and can even cause a dash fire. One guy here did that. You would need to extend the monovalve wires. For now, do step1 to get thru the winter. That might even keep the babe on your leash. If the relationship progresses, work on a used monovalve. One guy even replaced it w/ a manual on/off valve for winter vs summer mode.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans