A mirror reflective element sub-assembly for a vehicular exterior rearview mirror assembly includes a mirror reflective element, a heater pad and a mirror back plate. The heater pad is adhered to the reflective element with a light-transmitting aperture of the heater pad generally juxtaposed with a light transmitting aperture of a metallic mirror reflector of the reflective element. The heater pad is adhered to the mirror back plate with the light-transmitting portion of the heater pad generally juxtaposed with an aperture of the mirror back plate. A blind spot indicator has a light source disposed at a first end of a generally elongated passageway, with a second end of the generally elongated passageway passing through the aperture of the mirror back plate to attach at the rear of the reflective element. The blind spot indicator is associated with a blind spot detection system of the vehicle.