Our Cabrio doesn't have heated side mirrors, but my '99 passat did, and in that case the dial you turned to select which mirror to adjust (L or R) also had a setting for defrost (a box with wavy lines icon), perhaps it is the same for you?
the mirrors have a R and L button and a 4 way button on the door. the rear heated window button is on the dash, I was wondering if they share that switch but am not sure.
Hmmm... guess I don't know. I wonder if VW has the user manual online somewhere. (I'd be a bit surprised if the rear-window button did double duty though, I haven't seen that before)
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