A mirror reflective element sub-assembly for an exterior rearview mirror assembly includes a mirror reflective element attached at a mirror back plate, which includes a mirror actuator attaching portion established at the back plate by a plastic injection molding operation. A heater pad is disposed between the mirror reflective element and the mirror back plate. An indicator associated with a blind spot detection system has a light source that is activatable to emit light. When the mirror assembly is mounted at a side of a vehicle and with the light source activated, light emitted by the light source passes through a light diffuser and through an aperture of the mirror back plate and through a light transmitting portion of the heater pad and through a light-transmitting window of the mirror reflector and exits the mirror reflective element to provide a visual icon visible to a driver of the vehicle.