a locked radio, is a safety feature to protect radio from wrongfully being removed from vehicles. this also happens when the radio looses battery voltage.
all you do is call the any gm dealer provide them with the radio serial and model number
and they'll give you a code to input into the radio to unlock.
turn the radio on on enter the code, I don't think you need a second plug, if the radio has turned on.
leave the radio on for 1/2 or until display says code
then simply enter the numbers you get from the dealer using the key pad on your radio.