A method of forming a mirror reflector sub-assembly suitable for use in an exterior rearview mirror assembly of a vehicle includes providing a glass substrate having a first surface and a second surface. Glass is physically removed from a portion of the second surface of the glass substrate to form a curved recess locally thereat. The second surface of the glass substrate is coated with a mirror reflector via a vacuum deposition process. When used in an automotive exterior rearview mirror assembly, the mirror reflective element provides a principal reflector portion and an auxiliary reflector portion to a driver of the vehicle who views the mirror reflective element when normally operating the vehicle. A mirror back plate is formed in a plastic molding operation and a heater pad is disposed between the coated second surface of the glass substrate and a first side of the mirror back plate.