I couldn't find where this has been discussed before. Saw one old post referring to outside rear of car placement. Is this the only workable solution?

I recently changed out the radio on my wife's new Fiat 500 and recommendations on their forum said to place the magnetic XM antenna to the top of the mounting bracket inside the dash. I did this and there has been no problem with reception. Has anybody tried this or is the outside mount the only option? I just happened to think about this and my install appointment is tomorrow morning. Help.