A vehicular exterior rearview mirror assembly includes a mirror head disposed at a mounting structure and having a mirror casing and a mirror reflective element. An indicator module is disposed at a rear portion of the mirror reflective element and includes a housing having an indicating portion and a connector portion, a circuit board disposed in the indicating portion and having circuitry and a light source. The circuitry includes electrically conductive terminals extending from the circuit board and disposed at the connector portion of the housing. An electrical connector has a connecting end that is received in the connector portion of the housing to electrically connect to the electrically conductive terminals. An anti-backout element is disposed at the connector portion. With the electrical connector received in the connector portion, the anti-backout element is positioned at the electrical connector to preclude backing out of the electrical connector from the connector portion.