We still must identify your A/C system. Being in a entry level model I presume you have manual controls and not Climate control. If the car was made later than about 1981 then it won't have a ETR switch it will have an electronic control in the temp wheel - Do you have a temp wheel or a rotating knob?
If you have a ETR switch it is easily exposed by removing the lower panel passenger side and its roght on the face of the evap housing, covered with insulation.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician