"Is there more in the circuit for the t-stat sensor? "
BINGO !!!!
I hate electricity... If I were you , given what you said about last summer, I would wire up that fan where you could turn it on and leave it on when you are in the car. They tried to be too fancy on some of these things ( they may have worked when the car was new.. but trying to keep up complicated things may not be practical )... and it would not hurt to have that fan on all the time.

Just for the record... you need to check, clean and straighten the fins on your AC condensor and your radiator...and put new proper strength MB antifreeze here in the spring.. which is specifically when MB suggests you install it...