I don't believe they have any control other than the switch. They should be on when the control is not on either adjust location.
Don't leave them on full time or you may find they don't work anymore.
How have you checked to see if they are working?
If they are not working I would start by looking at the fuse and then the relay if any.
They should come on when the rear defrost is on or with the 'adjust' switch. The heaters on my '99 Passat didn't seem to work either, but the dealer found only the passenger side not working. So I don't think you're supposed to have really warm mirrors.
Never really felt my rear window when the rear defrost is on, does the whole window feel warm?
I swear I had the rear defrost turn off once when done, but some quick tests showed me there isn't a sensor. So I might have pressed the button again and forgot. The mirror heaters are known to overheat, I don't expect sensors here either. If you press the rear defrost button, the rear window and the mirrors heat up, if (for whatever reason it might be) you only want to warm your mirrors, you can use the adjustment joystick. That means there is no option to only have the rear defrost on (that option becomes available as soon as you burn your mirror heaters)
Want to talk about the cup holders? Most things work really nice, but they definitely have their 'flaws'. As long as it are just minor things, I'll live with it.
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