Below is a diagram of the 2003 Silverado diagnostic link connector (DLC) that explains how it works. As you can see here, only four (4) DLC pin terminals are used. Now you can fix non-communication problems in this vehicle model (even the 2004-2006 model) using a cheap voltmeter and scanner. Please review the details below.
Green lines above are all class 2 bus wires connected between DLC terminals and splice bar modules. You can get this exact wiring diagram from your local parts stores or get it online from ATS