I am convinced GrandAm would have much more fans if the cars looked better (something like the Fabcar). Not to like GrandAm because the green house is ugly sounds absurd but it isn't. Other series (touring cars, single seaters) are mainly about the drivers and racing itself but in GT's and sportscars the cars are much more important. In GT's and sportscars there's more emotion involved.
I think for Europeans the involvement of NASCAR isn't a big reason for disliking GrandAm. Here in Europe Formula 1 is the evil empire.
@Speed-King: The cars could even look worse