How can you tell if they are working?

When it rains I don't see anything happening.

Any fix for this?

 

Don't believe they were designed to clear or evaporate rain droplets. I've always used to defrost/defog.

 

I think it's to help de-ice the mirrors and get the fog off them, not sure if they would get hot enough to keep water off. The mirror tint can burn if the heaters are left on too long causing brown discoloration against the blue tint.

 

Whether or not they were originally designed to clear rain drops, they do in fact work to dry small rain droplets. On my 1hr commute to work if it's raining, I can get the mirrors about 90% dry using the heated mirrors button. I guess this is kind of a test you can perform?....

B/c even if you test for continuity @ the connector to the mirror switch and it tests good, if there's a break in the heating element inside the mirror, it won't function.