The mirror heaters are automatic, you dont switch them on yourself, they only come on when the temperature is around freezing, so in your case, using the heaters to dry off rain drops in temperatures above that they wont as they wont come on.
Many cars they come on when the heated rear screen is on, but with the MINI they dont.
I think you would be wasting your time with the standard fitments if thats what youre looking at? unless you can find someone to fit the heaters and make them work off an independent switch you operate??

 

ΤheNun how certain are you that R56 mirror heaters come on only at certain freezing temperatures?

 

The heated mirrors on the Mini are next best thing to useless unless is freezing, you cannot switch them on yourself to remove mist and rain ::frown::, cannot understand why they did not wire them to the heated rear window like other cars.

 

i thought they came on if you put on the heated front screen (if fitted?) hmm i might have to give it a test!

 

Theres no reason you cant wire them up to a separate switch and get them to work yourself though, if your that way inclined.

I found the best thing is to coat the mirror glass with something like Rain X so the water doesnt stay on the glass, it blows away as soon as you start moving.??

See page 34 of your 2008 drivers handbook re mirrors heating is automatic and only below a certain temperature.
http://libraryofmotoring.info/reference/manuals/

 

Fitting Heated Mirrors?

There is no pre-existing wiring loom in the doors for them either. It's easy enough to buy the heated mirrow glasses, but you also need to change the loom in the doors as well or wire them independantly.

I had heated mirrors and windscreen on my original 2002 MCS but although the screen worked fine (it's obviously switchable) I don't think the mirrors ever came on even when there was frost on the screen. I did wonder if they were connected up, or if they were even fitted despite specifying them at the time of purchase. I never managed to check this out with the dealer before I had to give the car back (it was a company car and I left the Co!).

Also I checked on my current 2003 MCS to see if it had heated mirrors, but just plain non-heated glasses fitted and no sign of any loom in the doors.