Whether or not the defrost vent cycles is determined by the outside ambient temperature and the position of the temp wheel. If it determines that it's cool enough outside to cause the window to fog, it'll move the vents around so that some of the air goes up the defrost nozzle to keep the window from fogging.

If the outside temp is warm enough (above 50 or so), it could be that the outside temp sensor is skewed in such a way that the CCU thinks it needs defrost.

There's a section in either the owner's manual or the FSM (I forget which) that gives a very detailed explanation of where air is sent and what conditions have to be met for it to do various things. It's a surprisingly sophisticated system, especially for 1980s technology. That's not to say that its without its own laundry list of quirks...

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]