How can i fix my radio on my S reg Nissan Micra?

I have had this car for just over a year. When I got it I bought a new CD/radio as the original was just dead. Everything has worked well up to the last 2 or 3 weeks, when the signal became weaker and weaker, I can only just get Radio 2 on it now, and nothing else. I took it to an audio repairer and he said the problem was with the aerial (the cd works find), but said he would need to take out the inside of the car to get at it, and he wasn't confident about doing that. I am not convinced that it needs such drastic action. If the aerial goes, I would guess it goes at the point it connects into the radio. He did try another roof aerial, which I had already done, and it did not make any difference. Can you help? Cheaply... the car is only worth a few hundred pounds, but it runs well and it is frustrating when you can't get reception. When it was first installed it could get long wave and medium wave, but nothing at all there.