Step 1: With car completely off, press start button once, w/o foot on brake. Puts car into accessory mode.
Step 2: With foot on brake, move gear shift from Park to any other position, then back to Park. Radio should come on, and last about two minutes. To start radio again, repeat Step 2.
FYI - If in Step 2 you shift from Park to Reverse, radio volume will reduce because Reverse started backup camera / object alert alarm. Volume will go back up, once gear shift is back in Park.