When you first turn your key to on, there are many lights on your dash that will light up momentarily, one of them being the CEL (check engine light), and that should go out after the truck starts. If you CEL lignt isn't on at key-on, it means that your truck has lost communication with the ECM. If you attach a code reader to your DLC (data link connector), it won't power up or read any computers. This may be because the cigarette lighter fuse has blown, or other shorts to ground in the circuit. If you can't communicate with the ECM, you can't load tunes.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator