Communications between vehicles constitute a Vehicular Ad hoc Network (VANET). Contrary to MANET, in VANET nodes which are vehicles can move with high speed and generally must communicate quickly and reliably. When an accident occurs in a road or highway, alarm messages must be disseminated, instead of ad-hoc routed, to inform all other vehicles. To position a broken vehicle (or vehicle in danger) and locate the vehicles in its vicinity is very important for the safety of the road users. However, vehicles are not necessary equipped with GPS and even they cannot obtain availability of line of sight access to satellites, particularly when they enter tunnels. In this paper, we propose an improvement to ODAM Optimized disseminating alarm messages protocol in order to support localization of GPS-unequipped vehicles. So, the number of vehicles discovering their localisation will be increased. This prevents pile-up of cars when fogs, accident or any other obstacle and then contributes to the driver safety. Analyses show that the optimal performances of ODAM can be reached even when the rate of GPS-unequipped vehicles is 40%.