Get the etadyne loopback connector to eliminate the Onstar, 03's have the analog Onstar system, no longer in use. So if the onstar unit is causing the headunit not to work, that should eliminate the problem. You can replace a head unit, but as I recall it must have been "Divorced" from it's previous car via Tech II, then re-assigned to the new vehicle. Hopefully someone will correct that if I'm remembering that wrong.
Here's an Onstar thread turned up by the infamous Search Forum function: