A mirror reflective element sub-assembly for an exterior rearview mirror assembly of a vehicle includes a mirror reflective element having a transparent substrate. A mirror reflector is disposed at a surface of the transparent substrate and a heater pad is disposed at a rear surface of the transparent substrate. A mirror back plate is disposed at the heater pad and has a first structure formed at a rear plate surface and configured to attach at an actuator of an exterior rearview mirror assembly. A blind spot indicator element is disposed at a second structure formed at the rear plate surface and includes a light source that is activatable to emit light. The blind spot indicator element and the second structure are configured so that light emitted by the blind spot indicator illuminates an icon so that the illuminated icon is principally viewed by a driver of the vehicle.