A mirror reflective element sub-assembly for an exterior rearview mirror assembly includes an electrochromic mirror reflective element and a heater pad disposed at a fourth surface of the rear substrate, with the heater pad having a substantially non-light transmitting portion and a light transmitting portion. A mirror back plate is disposed at said heater pad and has a plate portion having a rear plate surface and having a front plate surface facing the heater pad. The mirror back plate has a first structure at the rear plate surface that is configured to attach at an actuator of an exterior rearview mirror assembly. An indicator element is disposed at a second structure at the rear plate surface of the mirror back plate. The indicator element, when its light source is activated, emits light through the light transmitting portion of the heater pad and through the mirror reflective element.