Radio reception problem -- please help

Please help!!! I have a 2003 e500, and i and my dealer can't seem to fix my radio. My reception cuts in and out while I drive. When at a red light or parked, the reception is fine, but while driving, the reception fades in and out when it shoudn't. We replaced the head unit, and it still isn't working. If anyone knows why, or knows a way to fix it, or the reason it is happenning, please e-mail me at alber55@aol.com, or post a message. This is driving me nuts!!!!!!!!!!!!!