A novel design approach to a low-profile dual- resonant wideband backfire antenna for Vehicle-to-Everything (V2X) applications is proposed. The antenna is composed of a semicircular principal radiator, a 152 sectorial reflector and a 112 sectorial director all installed above a metallic planar ground. At first, a 2.0-wavelength prototype magnetic dipole is introduced to lay down the flared angle, radius and the usable resonant TM1, 1 and TM3, 1 modes of the principal radiator. In these ways, a wideband backfire radiation characteristic can be generated. Then, the multi-point equivalent source model is developed and used to predict the initial values of the sectorial reflector and director, e.g., the flared angles, radii, and the separations between the principal radiator and director/reflector. Prototype antennas are in final designed and fabricated to experimentally validate the dual-resonant wideband property on the 2 mm-thick air substrate, thereby exhibiting an impedance bandwidth of 27.6%. As also numerically and experimentally validated, the backfire gain can be further enhanced to 4.2 dBi. It is evidently validated that the proposed approach can be employed to effectively enhance the backfire radiation of low-profile, vehicular antennas.