A polariser might help some, but probably the best thing is to get one of the collapsible, flexible rubber lens hoods. You want something fairly wide angle and fairly long, if you can manage both.
Then hold the end of the lens hood in contact with the window (all the way round). This shades the window inside the hood from light from inside the vehicle, preventing reflections.
With a rubber hood, you can then angle the camera instead of having to be exactly square on to the window. (Bend it too much and the hood may start blocking some of the view, which is why a wider angle than needed is good - it
allows you to angle the camera more before you start seeing the hood in the picture.)