I though I was in for the same job with my 190E. Ever since I purchased it early this year I have been chasing coolant leaks. First was the coolant expansion reservoir and then the radiator. The remaining one was the coolant that dripped from above the transmission and the coolant odour through the vents when the heater was operating. Then I noticed the heater control valve leaking (near the wiper). A new heater valve and no more coolant odour, and hopefully no more coolant leaks. I didn't fancy having to replace the heater core, especially after seeing your pics.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.