Top of the roof works the best. On the dash may get a signal, but since it has to go through the glass and at some angles (depending on where the nearest satalite is to you) the roof may be in the way.
Usually the satalite is not directly above you. So you have to keep that in mind when you're finding a location, since the signal can't go through much of anything.
There's unfortunatly only a few satalites over the US (depending on who your provider is), which makes it kind of crappy in some areas no matter where it's located on the vehicle.