Make sure you're using the right protocol... There are 5 or 6 different protocols so you need to use the right one for your vehicle. GM only uses one of 2 protocols, most everything is CAN based now but back in 2000 some vehicles were still the other protocol. That may explain not being able to connect to the vehicle, and the security light coming on. Could be completely unrelated, but it's worth checking into.

The standard OBD2 connector is used with all of the different protocols, so my WRX that uses the ISO9141 protocol uses the same connector that your Silverado uses for CAN or whatever GM used back then on that truck (J1850 probably?).