I have a 97 bmw 528i and the stock head unit. Sometimes the stereo goes completely out. I'll start the car and nothing audio related works at all. The display still works, but pressing any audio related button does nothing, clock works fine still. It's done this before, and will stay off anywhere from a couple hours to a week, then randomly start working when I start the car again. It's not a bad head unit because I swapped it with the same one in my fathers car and still nothing. I've checked all the fuses and everything seems fine. I'm at a total loss for how to fix this.