Applications aimed at enhancing the experience of vehicular transportation have been increasing in recent years with the widespread diffusion of smart mobile devices with network capabilities and access to user location. Such applications include navigation systems and location-based timetables. However, most of these applications only use individual contextual information in order to provide useful services to the end user. Sharing contextual information with other users can open a host of new possibilities, such as providing live traffic monitoring, where the location and the speed of individual cars are shared and indicates the flow of traffic; or friend locating, where the location of friends can be displayed on a map. In this paper, we argue that there is a lack of specialized programming support for such applications and we present DYMES, a dynamic messaging service devised to help fill this gap. Central to DYMES is a dynamic publish/subscribe system, which allows the publication of dynamic contextual information and the creation of dynamic context-based message filters. We present the core APIs provided by DYMES and illustrate their usage via two typical VANET applications. Furthermore, we identify and discuss implementation issues which guide the architectural choices in our ongoing work.