In this paper, we proposed a novel routing protocol called Geographic Member-Centric Routing (GMR) over a VANET that consists of a platoon of vehicles, which have the same starting point, driving route, and destination point, driving in the highway environment. The goal of GMR is to aggregate 3G/3.5G bandwidth from some vehicles called Helpers to a vehicle called Requester, i.e., it belongs to multiple-source single-destination transmission, to have cooperative video streaming service in the platoon of vehicles. For providing a better cooperative video streaming service in the platoon scenario, we (1) utilized the characteristic of platoon mobility, in which platoon members try to drive with each other as close as possible and (2) proposed a member-centric scheme that routes packets through more stable links between platoon members. We also introduced a concept of flow-merging, using which k-hop nodes would forward packets to 1 or more merge-nodes in (k-1)-hop, to reduce interference and packet collision between multiple flows. According to the simulation results, GMR effectively improves the packet delivery ratio and throughput using our proposed member-centric scheme and flow merging scheme. It also shows that the proposed member-centric routing protocol is more suitable than others for 3G/3.5G bandwidth aggregation in the platoon scenario over VANET.