It will be either the antenna or the tuner. Analogue Modulation (AM) signal arrives differently from the way Frequency Modulation (FM) does. AM has a much greater range, and is a better quality signal during the day. At night the atmospheric conditions are different, and so more signals can be received from further away. These tend to bleed over each other and so the reception is very poor. FM signals have a much shorter range and come direct from the transmitter, and so are not affected by the change of night and day.

A bad antenna connection or a faulty antenna amplifier could be the cause. Does the tuner lock on to a station when seaching? ie. Radio One which broadcasts between 97 - 99 MHz, or Radio Two 88 - 90 MHz. If not I would suspect the antenna. Have you tried substituting the antenna? If you have and it still doesn't work then you almost certainly have a dead tuner.

Plenty for sale on Ebay but I think the 4500 is the one that's a bit of a pig to change.