I have a 2001 Yukon XL. The heated mirrors to defrost during cold weather have quit working. The indicator light on the button which switches on the mirror heaters and the rear hatch defroster lights as if power is being sent to the mirrors. The rear hatch defroster still defrosts. I have checked the fuse for the mirror heaters and it is OK. Any ideas of what is causing the mirrors to not get power to their defrosters? Thanks, Chris