Wave propagation in tunnels for vehicle-to-vehicle (V2V) communications scenarios is characterized by multiple diffuse reflections on tunnel surfaces as well as specular reflections on other objects inside the tunnel, leading to a nonstationary fading process. Such a fading process is difficult to model by ray tracing (RT), requiring a prohibitively high computational complexity due to the large number of diffuse reflections. In this paper, we propose two new ideas for modeling diffuse reflections in nonstationary scenarios: 1) We partition the nonstationary fading process into multiple stationarity regions with a given extent in time and frequency for which approximate wide-sense stationarity can be assumed; 2) we propose a hybrid model, tightly interlinking RT with a propagation graph, such that vertices for the propagation graph are obtained from interaction points calculated by RT for each stationarity region. We compare our hybrid model with measurement data in terms of the time-variant power-delay and the Doppler-power spectral-density as well as the root-mean square delay- and Doppler-spread. This analysis shows, that our hybrid model is the first numerical simulation model that is able to model diffuse reflections inside a tunnel with correct nonstationary (i.e., time-variant) temporal correlation for a nonstationary V2V communication link.