Without RPO U88
WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
No audio output from the radio-Test for any AC voltage between the left front audio signal circuits at the left front speaker connector with the radio volume adjusted to 50 percent. If no AC voltage is present, check for shorts to battery positive or shorts to ground on any speaker output circuit from the radio. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. If no shorts are located, replace the radio. Refer to Control Module References in Computer/Integrating Systems for replacement, setup and programming.