You should not allow any part of the gun to be concealed by your clothing.
While it states it needs to be in "plain sight" the court case was referring to a weapon on the seat, if I recall. So long as your clothing, and I think a seat belt not covering it would also be a good idea, isn't covering any part of the gun, you should be fine.
I open carried to the Sheriff's Department Concealed Carry class and had zero issues. I have open carried since July 2010 in Idaho and haven't had a single issue.
I wear a holster where my sweatshirt doesn't even accidentally cover it. I also am able to tuck the seat belt under it.
Just make sure that if an officer comes to the vehicle, they would be able to see it in plain view.
Idaho is pretty awesome in the regard of having officers that don't freak out because there is a firearm, as far as I've seen.