My screen for my GPS and radio went black and the radio will shut off repetedly.

Hello. I just bought my bmw about 4months ago. Had the same exact problem. Blank screen and radio turned off after couple minutes. After that it wouldnt start after couple cranks. One Saturday it just didnt start at all. Changed the battery and came to find out that the the car didnt have the correct size bettery.Bought it and put on. Worked ok but sometime I had to crank it couple time before starting. Decided to take it to my regular mechanic to change the belts, thinking it might solve the problem. My mechanic called back. His diagnostic was that the tensioners were going bad and my alternator as well. Ok'd the replacements. Got the car back and to my surprise the radio was also working. I didnt even tell him about the radio problem. This mechanic is the most AWESOME AND HONEST MAN. If he can fix the problem without replacing the part he'll fix it(not in a mickey-mouse way). I've been bringing my cars to his shop for 11 years. Never had a problem. He's located in Gardena Ca. Name: LAX Mobile: 14301 S Western Ave, Gardena Ca 90249