Thanks everyone!
jrd.racer - Use the background given to cover the spots you don't want to see. Nothing is warped, I just copy/pasted some parts of the background on the left and right side to cover the places where the car was seen and I did the same thing to make the background behind the windows. If you look closely, you'll find the same stuff behind windows as on the left side of the car, just copy/paste things and use eraser with low opacity to erase the edges that usually make a clear outline to be seen, once you've erased them there is no outline and it looks good.
About the windows: I select the spots which I need to be visible, I cut them to a new layer. Then I copy and paste multiple times the same layers so that there wouldn't be any outline around the selected areas (which it usually makes). Then I take a color from the seats or whatever was close to the window to make a similar impression as the previous one. I fill the cutted parts with that color, change the opacity to ~60% usually and it's done.
You probably didn't understand a thing I just wrote, well, if you need some more help, just ask in PM and I will try to answer in a more detailed way.
You can have a look at some old tutorial I made how to change background, although it's in Latvian it is also shown with the pictures what to do:
http://www.lfslatvia.lv/forum/viewthread.php?thread_id=351 (About the windows it starts from point 5. Although I do it in a little bit better way nowadays, it is very similar and you should get the point.)