Tutorial: Opening the windows!
The easiest thing you can do to a car.. for quite a big effect..
 Opening the window
Difficulty - *
Must be getting a bit warm in that BMW of your's mate? Lets open the window!  
Lasso tool is used again this time. go around the top of the window and around much like I have done in the picture.  

Right, go to Image>adjust>Brightness/contrast, and lower the brightness and contrast to the same about the same settings as I have done.  
Looking ok, now we need to add some form of highlight to the top of the window.

Create a new layer.. and draw a white line 1 Pixel in size all the way around the top.

Filter > Blur

And then lower the opacity to suit the picture.