Hi,
Did the poor reception happen right after you installed the new stereo? If so check the radios antenna connection on back of the radio again to see if everything looks ok. If it looks good, I would try plugging in the old radio and see if the reception comes back in strong again. If it does, then you may possibly have a bad new radio. How does the radio work on AM mode? If strong on your new radio, I would say the antenna connection is ok, and that the new radios FM portion is faulty! Hope this helps you out.

Jim LaPlant
Factory Trained Mercedes-Benz Technician