Two options that I know of. 1 being, if your truck has a tape player you can buy a cassette adapter from any walmart, radio shack, best buy, or pretty much any place that sells audio equipment. It plugs into the headphone jack on your iPhone, and plays through your tape player. If you don't have a cassette player in your truck you can also buy an FM converter. Its more expensive than the cassette adaptor, but can be found at the same places. That also plugs into your headphone jack, but has a digital setting that you set on the device to a radio station not in use in your local area. You then tune your radio to that setting, and your music plays through that station.
In order for you to play your iPhone in a 2004 Chevy Silverado, you will be required to turn your key to the on position to power on your Chevy's electronics. Attach one end of the cable into the audio jack on your iPhone. Press the CD or Aux button on your radio. Check out more on http://www.ehow.com/how_5912121_use-chevy-aux-input.html.