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bear with me on this one guys - Having just moved back to UK from New York, where we didn't have to use a rear window demister due to hardly any moisture, so no frost or dew in mornings, I am not so sure my rear demister is working on my car. For example, this morning there is dew on the car so you cant see out the rear window - turn on rear demister, the light comes on but it doesn't seem to make any difference.

Should it work when the window has dew on it or do i have to wait for some frost to test it

Would it be slightly warm to the touch if on (as a means of seeing if it works ok)?

Dunc

The rear de-mister is designed to remove condensation / mist from the inside not the outside.

The very small amount of heat it generates, is however enought to melt any frost that appears on your rear window.

If you have a saloon or Avant with standard (non-foldable mirrors) and you put the rear demister on it should also demist the wing mirrors. Leave the switch on and then touch the glass on them, it should be warm to the touch.

If not working then it may be the fuse, if this does but you feel the rear is still not working then it may be a connection. Quite hard to tell with just dew, can take up to 10 mins.