Just yesterday got my car back from the shop. Had brown fluid in the coolant tank, thought it was oil, but now think it was ATF since just last week the plastic side tank of my radiator sprung a BIG leak while idling in my driveway, spewing coolant everywhere. The lower 1/5th of the 124 radiator is a transmission cooler, so if there is deterioration it is possible for ATF and coolant to intermingle. Seems the radiators in 124s, at least, are almost considered a normal wear item. Even with a reinforced neck (which mine had), the plastic tanks on each side seem to get brittle with age (and I have always used MB coolant). (Mine has 99K miles on it.)
On another list I recall reading that some recommend prophylactically replacing radiators on some models at 100,000 miles. Now I know why. Just glad mine didn't go kerflooey on I-80 in 95 degree heat.