The antenna must not be shielded by being too close to, or behind, the sheet metal. Sometimes stretching the wire under the dash pad works. I hooked an AM antenna up(wire coiled around a plastic oval like on a home stereo)on my Crown Vic and stuck it up under the dash. Some people make something to sit on the dash to hook it to. It's like the radio antenna, if it was inside the car, it wouldn't work very well. Of course the more expensive ones work better but they all have this problem to some degree.
Oh, and make sure all of the grounds on the unit are grounded (shorter is better).