Pervasive vehicle-mounted mobile devices are increasingly common, and can be viewed as a large-scale ad hoc network on which collaborative, location-based services can be directly supported. In order to support such services within a geographic region, a certain number of computational, storage and sensing mobile devices need to be carriers of the services. This paper introduces and evaluates Region-Resident Services (RegReS), a middleware that supports such regional services by maintaining, in a fully distributed fashion, a targeted density of service carriers. Carriers collaborate opportunistically to estimate the current service density in the region and coordinate the spawning of new service carriers when necessary. Unlike previous approaches that are static, RegReS adapts to dynamic conditions such as node speed, effectively maintaining the targeted density of service carriers in highly volatile vehicular networks. Results from the ORBIT testbed, using synthetic and real bus mobility traces, show that RegReS adapts to different system configurations, preserving the desired service density with less than 16% mean absolute error. We deployed an outdoor collaborative parking availability service atop RegReS and demonstrated RegReS's ability to maintain the target service density with only 10% error.