Keeping Memory

here is my problem i have a friend who has a 911 porsche 1990 who drives it every blue moon what i want to do is something to keep the memory on the unit so she doesn't have to input the code the unit is a cdxm700r (the 2 faced sony deck) and yes you guessed it she has lost the remote which you need to input the code to unlock the unit also the place were she stores it has no ac outlets so a plugin slow charge unit is out of the question so the challenge is set any help will be much apprieciated I Love This Industry!!