Security mechanisms recommended by VANET standardization bodies are not sufficient to stop authorized vehicles from sending bogus or fake messages that in the worst cases could put people's life at risk. There are efforts in evaluating the behaviour of the vehicles in VANETs, based on reputation mechanisms. However, the most of them compromise the privacy, the messages are linkable and no anonymous. Our proposal ARS: Anonymous Reputation System for VANET focuses on a centralized reputation system privacy-preserving using pseudonyms and giving reputation level to different behavioral aspects of the vehicle. A reputation server collects feedback from the vehicles through a Road-Side Unit or vehicles with LTE/4G/5G connection and updates the vehicles reputation level by matching the pseudoidentities with its real identity. Considering the amount of negative feedback, ARS defines two security states to adapt the applications to the real security risk. We implement and evaluate the performance of ARS through a set of simulation experiments, based on an opportunistic message forwarding application. Results showed that the calculating reputation level of vehicles could help reduce the dissemination of fake messages.