The lower radiator hose is looking like it's in need of replacement, so that will be a good time to do the thremostat also. Thanks for cluing me in on the location of the t-stat. I thought it was at the upper hose.

Since topping off the coolant, it hasn't gotten much above 80*, but I do plan to investigate the fan further. On my '81 300 SD, where is the 3 prong switch that you show the picture of located? In re-reading this thread, I think it's by the upper hose, where I originally thought the t-stat was. Am I correct? Is accessing it and changing it out as straight forward as one would imagine?

Before I do much, I want to further investigate where the coolant is going. With my next oil change, I plan to send a sample for analysis to see if it's a cracked head. I know the system is holding good pressure, as even after the car has sat overnight the radiator cap comes off with a "POP."

I find it odd that whenever I look at the bottom of the overflow hose, there is fresh coolant dripping out. Only a drop or two when I look, but it is visibly moist with coolant. As an experiment, I have just place that overflow hose into a plastic bottle in an effort to see how much is collected over the course of a few days.