In a scene where Jimmy Stewart is in a cab a cab in the rear window following them is a red cab (Ford I think). However, when he exits the cab there is another color car behind his cab and it's a different manufacturer than the one shown in the rear window. A few minutes later when Steward gets back into the cab, the same red cab is in the rear window. I pointed this out to my father and he was the one who told me the car manufacturers but I don't remember exactly what they were.