Not really a problem; just thought I'd share.
It's been fairly rainy the past couple of weekends, and of course, the rear window has fogged up several times. But the rear window defogger seems to work v-e-r-y slowly. Hard to tell because once I turn on the air for the windshield, it circulates and voila! No more foggy rear window.
I stopped at my smart dealer the other day and mentioned this. I was told that cabriolet rear windows are particularly slow to heat, possibly because the manufacturer is treating the top gently. It often takes several cycles for the defogger to do it's job properly.