Thermostat should be closed at freezing temps and open at warmer temps, usually somewhere around 40-50˚. Reset temp is typically somewhere around 20-30˚. The element should always come on when defrost is initiated if the freezer has been running. The thermostat opens and terminates defrost when the coil warms up (frost melts off).

Repeated freezing of the coil can be low refrigerant charge (especially if the coil is not evenly frozen), bad defrost timer (very common), leaking door seal or something stuck in the door preventing it closing, or something blocking the intake vent area in the freezer section (bagged foods for example).

__________________
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]