Your truck has two antennas because they are designed to pick up different types of radio signals. The main antenna is the one located on the roof and it's used for AM/FM radio stations. It is usually a short antenna, sometimes referred to as a shark fin antenna, and amplifies reception in order to give you better sound quality and more range when driving around town or out in rural areas. The second antenna is often found near the rear window and it's used for satellite radio services like SiriusXM or HD Radio. This type of antenna may be longer than the first one as it needs greater reach to access these signals from space-based satellites or transmitters on tall buildings. Having both antennas improves your chances of getting clear reception wherever you drive so that you can enjoy your favorite music without static interference.