Side mirrors

[Tridaddy]

I have a self dimming, turn signal integrated, power folding driver side mirror with puddle light on my 04 Yukon XL SLT. I have been searching for a replacement glass/ entire assembly. The salvage yards which are contacting me say I should have heated mirrors also. If turning heated mirrors is done manually, I do not have an independent switch for it. Even if it is automatic, I thought there was a lighted symbol which would appear on the mirror.
So, my questions are:
-How do I know if my mirrors are heated?
-If they are not heated, if I find a mirror which meets all my other requirements as well as being heated, can I use it without messing anything up?
-If I find heated which meet my requirements plus being heated, can I make them work as heated mirrors by adding a factory switch to my vehicle without much else?

Thanks.

 

[Timmy]

heated mirrors are turned on via the rear defrost switch

 

[Tridaddy]

Thank you. That makes all the sense in the world now. It actually worked this morning; there was a heavy frost, I turned on the rear defroster and watched as my mirrors cleared up.
Now, to get my replacement mirror.