Heated Side Mirrors

[TS135i]

I read somewhere the heated side mirrors automatically heat up when below a certain temperature. I thought they automatically heated up whenever the rear defrost button is switched on, anyhow being summer the heated mirrors don't seem to function even though the rear window defrost heats up. The side mirrors still have condensation and takes forever to dissipate.
Is this because the outside temp is above the threshold? I didn't have this issue in winter...