BMW Repair: 1991 525i Electrical, eletrical system, aftermarket radio

Question
Why would the eletrical system seem to malfunction, such as moon roof won.t open, door locks fail to operate, radio failure and then just as suddenly everything is OK.  This has occurred 3 times for my daughter and everytime I check it out everything is fine?

Answer
it could be the control unit that control window and door lock functions. the radio is probably a separate issue. do they all happen at exactly the same time? does the car have an aftermarket radio? sometimes when aftermarket radios are installed, they are installed poorly which can create other problems