Cooperative communication has been recently applied to vehicular networks to enable coverage extension and enhance link reliability through distributed spatial diversity. In this study, the authors investigate the performance of a cooperative vehicular network over a doubly selective (i.e. frequency-selective and time-selective) fading channel. Under the assumption of amplify-and-forward relaying with orthogonal cooperation protocol, the authors derive a pairwise error probability (PEP) expression and demonstrate the achievable diversity gains. The authors' results demonstrate that, through proper linear constellation precoding, the cooperative vehicular scheme is able to extract the maximum available diversity in frequency (through multipath diversity), time (through Doppler diversity) and spatial (through cooperative diversity) dimensions. The effect of imperfect channel state information is also studied through an asymptotical PEP analysis. The authors further conduct Monte-Carlo simulations to confirm the analytical derivations and present the error rate performance of the vehicular scheme under various mobility conditions and scenarios.