An exterior rearview mirror assembly for a vehicle equipped with a blind spot monitoring system includes a mirror reflective element sub-assembly having a mirror reflective element, a mirror back plate and a heater pad disposed between the mirror reflective element and the mirror back plate. A light transmitting aperture of the heater pad is juxtaposed with a light transmitting aperture of the mirror reflector of the reflective element and with a light transmitting aperture of the mirror back plate. The mirror back plate includes first structure that is configured to attach at an actuator. A blind spot indicator is disposed at the light transmitting aperture of the mirror back plate, and, when activated responsive to a blind spot monitoring system of the vehicle, emits light through the light transmitting apertures, forming an illuminated icon viewable by a driver of the vehicle viewing the mirror reflective element.