85 rear window defogger

I have had an 85 coup since january. The rear window defogger only stays on for about 20 seconds. How is the timing determined? Would replacing the switch fix it or is it just a bad design?
Thanks
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JimS wrote:

Is it a consistent 20 seconds? Could it be that once the wire gets warm there's a break in it somewhere?
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Yes, its almost always 20 to 30 seconds. If I disconnect the "grid", the heated mirrors stay on indefinately.
Thanks for the reply

on
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