From the DVD commentary, episode 3:
Douglas Mackinnon: Little bit of magic. If you’re careful and you look at the beginning of that shot you’ll discover that Aziraphale isn’t in the car. And then suddenly he is.
Neil Gaiman: Did we magically CGI him or was he just sort of bending down?
Douglas Mackinnon: We CGIed him out. And the other thing that we should know about this car is that the morning that we shot this we - well it was David Tennant - broke the car. He broke the door on Aziraphale’s side, so you couldn’t get in or out of it anymore. It was stuck in, and there was actually a broken window.
Neil Gaiman: Which we had to CGI out.
Douglas Mackinnon: Yeah. So, at the end of the scene - although Aziraphale appears to get out of the car - he can’t.
Neil Gaiman: Because he couldn’t open or close the door.
Douglas Mackinnon: It was a directing cleverness to get him out.
+ how did it happen? (not really David’s fault :))