How to Unlock 1999 Mitsubishi Eclipse GST Infinity Radio?

[GeforceXtreme]

Well I replaced my friends sisters 02 sensor a few weeks ago in her 1999 Mitsubishi Eclipse GST. I unplugged the battery to reset the error code and when I did this now the factory Infinity head unit is now locked out. I told her to look in her glove box to find the unlock code however of course its not there. I would like to find out if there is a method to unlock this radio without having to spend $80 dollars at the dealer. I was told that if I took the factory radio out and get the serial # that I could call the dealer again and they may give me the unlock code for free? However I don't want to do all that work for nothing. I also am unsure of how to remove the stock radio.

There must be an easy way to do this because her sister took her car to get the battery replaced at BJ's Wholesale Club not that long ago and they most likely would have had to unlock the radio somehow?

Any help with this guys would be greatly appreciated!