a mounting adaptor is disclosed for removably securing an interior rearview mirror assembly to a mounting support on the inner surface of a vehicle window such as a windshield . the adaptor preferably has a recessed channel on one side for receiving a window mounting button and a projection on the opposite surface simulating a window mounting button for removably mounting a rearview mirror assembly . the projection may be shaped to receive the same or a different mirror assembly channel mount as compared to the support on the window to allow conversion from one assembly to another without changing the window support . the adaptor may also include a support for a vehicular accessory such as a microphone , compass , headlight dimmer , light , moisture sensor , radar detector or the like . such accessory is thus supported for convenient use by the driver or other vehicle occupant and in a highly advantageous operating position without interference with the rearview mirror , its replacement , interchangeability or adjustability . a wire cover may also be included on the adaptor . an assembly combining a microphone and a support for mounting a rearview mirror adjacent a driver within a vehicle is also disclosed .