When the Climate Control system is left in Auto mode the system will turn on the AC as needed, i.e. when it detects that the ambient temperature, air coming into the car, needs to be cooled in order to reach the correct temperature dialled into the control unit. You can tell if the system has turned the AC on as the orange light will be lit under the AC button.
Now at this time of year the system keeps the AC off, even in Auto, which is a bit of a pain as when it rains the car tends to steam up due to all the moisture in the air.....soooo to avoid this happening you have to engage the AC manually which will disable the Auto funtion. (AC removes the moisture from the air and hence prevents the car steaming up)
On my 206 GTi whenever Auto was engaged on the Climate Control the AC was engaged as well. Id prefer this on my 182, however it just means that when I manually engage the AC in the rain I have to dial in temp/fan speed/exit points etc. rather than the system doing it all for me.
Cheers Paul
Ps. When you engage the demist function AC will also be engaged.