A mirror reflective element sub-assembly for an exterior rearview mirror assembly of a vehicle includes a mirror reflective element and a mirror back plate. The mirror back plate includes an indicator unit mounting portion and a mirror actuator attaching portion established at a first side by a plastic injection molding operation. A heater pad is disposed between the mirror reflective element and a second side of the mirror back plate. An indicator unit has a light source that is activatable to emit light and is mounted at the indicator unit mounting portion. Light emitted by the light source, when activated, transmits 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 be viewable by a driver of the vehicle equipped with the mirror assembly.