The CCUs are known problem components for 107s, 126s - basically any car with the push button/thumb wheel climate controls. They just wear out. What happens first is the solder joints between the thermostat wheel and the fan controls become intermittant and finally they just quit. Problem is that it is intermittant - so you can drive yourself CRAZY trying to diagnose.
The Benz Store out of Georgia has rebuilt CCUs with a 3 year warranty for $135 - $155 (depending on model) + $45 -$75 core. Best money I spent - got a working AC, vent control came back and shiney new buttons!
If anyone is having AC problems, replace the CCU first! Cheap insurance for something that is known to be problematic.