With the car off press and hold the power button with out pressing the brake. I want to say 5 seconds, but it could be longer or shorter.
At least turning Volt all the way OFF and opening the door should work and you can test to see if the radio drops out as well.
thank you all very much !!Check your Volt owner manual under the Driving and Operating section. The Power Button portion near the start of this section speaks about turning the vehicle on and off, and Service Mode ("With the vehicle off, and the brake pedal not applied, pressing and holding the POWER ON button for more than five seconds will place the vehicle in Service Only Mode"), and a page or two later it speaks about Retained Accessory Power.