We use them all of the time, this is what they are made for.  The element
warms the glass slowly so there is no problem.  Mine is almost always on in
my van.  If you hit the glass with a sudden blast of heat it would break but
a gradual warming is fine.  I worked in a Ford dealership years ago and a
lot attendant sprayed a row of cars cold with hot water from the wash truck,
took out every windshield and they were all the then new style heated glass
at around $1,000.00 each.

Mark in AK

On Fri, Dec 11, 2009 at 4:36 PM, Rob <becida@comcast.net> wrote:

> At  12/11/2009 02:08 PM,Matthew wrote:
>
> it was around zero degrees f here this week. A coworker claims that
>> if i use my rear defrost it will break the rear window? Before i
>> call Tom and Ray, will someone chime in??  it seems a bit improbable.
>>
>> thanks
>> Skeptic Matt
>>
>
> That's what the rear defroster is made for, never had a problem with
> it in sub zero temps on several different cars.
>
>
>
> Rob
> becida@comcast.net
>