Transformers

I recently bought the movie Transformers. Before purchasing it I had not seen it, I bought it purely on the good reviews I got on it from my friends. The movie itself was not bad but I was surprised at the amount of product placement. I knew that Chevy was a big part of the film but they may have went a bit overboard. the thing i do not understand about it though is how Chevy got the right to be in the movie over Ford (just for arguments sake)? was there a bid or did the producers of the movie seek out Chevy? This summer there was a ton of Transformers/ Chevy advertisements, that I personally do not think were done very well, were they a part of the original agreement? Does anyone know how this whole situation works? I would really like to know.