Initial works on cooperative vehicular communications build upon the assumption of frequency-flat and quasi-static fading channels. This can be justified only for narrowband systems in very slow traffic flows such as in rush-hours. In this paper, we consider doubly-selective (i.e., time- and frequency-selective) vehicular channels and investigate multipath-Doppler diversity for a cooperative vehicular system to extract the underlying rich diversity. Our performance analysis through pairwise error probability derivation shows that, through proper precoding, the proposed system is able to extract maximum available diversity in time, frequency and space. Monte Carlo simulations are further presented to confirm the analytical derivations and corroborate on the analytical results.