FAULTY AUDIO UNITS?

WHAT can i do about my 2008 fiesta Van 1.6 sport,TDI faulty audio system. i have taken it back to a dealer twice. each time they cant find a fault -they are now changing the radio again? the problem is the audio system turns on and off at random. in the middle of the night the radio sometimes turns on.... and when i unlock the car and push the power knob -nothing happens. the only thing that works is the volume. so i have to remove the main audio fuse. this happens a couple of times a week. also turns off when driving and then turns itself back on 5 mins later. Ford cant find a fault.......... any advice?