It really does depend on the vehicle. Some people I know can't get a reading inside their vehicle at all because of specially designed windshields that have some kind of metal in them. Generally, though, your best reception is going to be with a clear view of the sky. Soft tops will cause a very slight reception problem, but probably not noticeable. On the downside of those air-vent holders, they will tend to break the vents after a while, and during winter months, you don't want hot air blowing directly on any electronic item. Heat and electronics don't mix well.