In the beginning of the movie when Cheech goes to his car, the window that says "LOVE MACHINE" is already rolled up. But when he gets in, he rolls it up to reveal the "LOVE MACHINE", back window.
Actually, if you watch the beginning closely, you can see that "Love Machine" is on both back roll-up windows. So the "Love Machine" that you see while he is cleaning the car is backwards, and when he rolls up the driver's side window, you can see both of the slogans!
Although not in the movie, but the original car was supposed to say "La Bamba" (listen to the earlier albums) and the movie was actually shot with the original "La Bamba". They removed the window and replaced it with a "dummy" window with "Love Machine" painted on it. That was Cheech's real car and didn't want "La Bamba" scraped off. Hence the replacing of the window. Must be nice to be rich, huh?