1987 325e will not start

[325e_mom]

I bought this car for my son and now I am ready to get rid of him and the car!! The car had starting problems when I bought it and after 3 mechanics, we finally got it to start consistently. They replaced the battery and all the belts. They tested the other components and said they were all were fine. After that, the car started and ran fine until my son and his friend tried to install a new cd player. Now when you try to start the car it does nothing. No lights, no radio, it doesn't even attempt to turn over; it does absolutely nothing. The battery is fully charged and all fuses are good. What could they have done and can it be corrected? Any suggestions would be greatly appreciated.