1997 Bmw 740 97 740il Radio Works Every Months Acts Loose Wire Does Work Only

97 740IL RADIO WORKS EVERY FEW MONTHS. ACTS LIKE A LOOSE WIRE WHEN IT DOES WORK IT ONLY WORKS UNTIL YOU SHUT THE CAR OFF THEN IT MAY BE MONTHS BEFORE IT COMES BACK ON. THE CASSETTE DOES NOT WORK EITHER AND THE FACE'S ARE DARK WHEN IT IS NOT WORKING IF ITS WORKING AND I HIT THE BRAKE PEDAL OR TURN ON THE LIGHTS IT QUITS. IF I HIT A BUMP JUST RIGHT IT WILL COME ON SOMETIOMES