You have a duovalve on a US spec W124? I thought they all had single channel heating. And the model specific monovalve is prone to cracking. I've heard of the coolant line in the washer reservoir rupturing and filling the washer reservoir with coolant but not the other way around. If there's no engine oil or ATF in the coolant, maybe it's just running hotter these days.

Soxto
83 300SD
98 E320 wagon