When operating from a vehicle, the vehicle can actually block your signal if your antenna is inside. Hanging out the window is not always practical or safe, but a 1/4 Wave Mag Mount Antenna mounted in the middle of the roof will likely eliminate that issue, especially when relatively close to those you are communicating with. Just run the cable through the door (pinching it in the rubber is normally not a problem), but make sure you have the appropriate adapters to connect the radio to the antenna.
Note: You generally need only a 1/2 or 1/4 wave Mag Mount Antenna, unless you are packing spare or loaner gear. A 1/2 wave will give you better performance, but a 1/4 wave packs down small, and will generally give you adequate performance for medium distances.