A mirror reflective element sub-assembly for use in an exterior rearview mirror assembly of a vehicle includes a mirror reflective element, a mirror reflector and a mirror back plate disposed at the rear side of the mirror reflective element. A heater pad is disposed between the mirror reflective element and the mirror back plate and has a light transmitting portion that is aligned with an aperture of the mirror back plate. A blind spot indicator module includes a body portion and a connector portion extending laterally from the body portion. Light emitted by at least one light emitting diode passes through the light transmitting portion of the heater pad and through a light-transmitting portion of the mirror reflector and exits the mirror reflective element to provide a visual icon visible to a driver of the vehicle who is driving the vehicle and who is viewing said mirror reflective element.