This paper presents a gain enhanced wide azimuth beam automotive antenna using half-mode substrate integrated waveguide (HMSIW) cavity for automotive applications. The antenna is separated into two HMSIW cavities by electrical via walls. The two HMSIW cavities are excited through differential feeding, including a 180 degrees delay line in the center of the antenna. The antenna was operated in TM11 modeand investigated using aperture theory based on the cavity model. The antenna size is small enough to mount in the rear-view mirror, which has a long rectangular shape. For comparison, the proposed antenna and a conventional 1 x 2 microstrip square patch antenna array were fabricated to the same dimensions. Then, these antennas were tested to provide validation. The tested results were consistent with the simulation results. The tested results show that the antenna, with the overall dimensions of 70 mm x 35 mm x 1.52 mm (1.38 lambda(0) x 0.69 lambda(0) x 0.03 lambda(0)), operates at 5.9 GHz. The gain of the antenna is 9.49 dB, and the azimuth beam width is 74.0 degrees. The proposed antenna has a wider azimuth beam width (74.0 degrees) than a conventional 1 x 2 square patch array antenna (42.0 degrees) with similar gain and size.