You know, I can honestly say that I have never found one that actually worked. But, with the advent of the new all digital signal, those days will be over. I put a 27" Samsung LCD in my Volvo, and it can actually pick up the new digital channels. No analog, but the digital comes in loud and clear over my factory truck antenna.
If you are using an old CRT (tube) television set, get a converter box and just plug it in to your existing factory tv connection. I think you may be pleasantly surprised. You will need a digital tuner anyways with a tube set, get one before you start buying antennas that will be obsolete in another month. Or not work at all.