Problem with radio?

I have a Peugeot 406 fitted with a CLARLON radio/cd. Model no. pu-2473A(B). Serial no. CLO54820041181. The CD is fine but the radio will not work. I have put a test radio in to check that it is not the aerial - test one works fine. For many months the radio would crackle and hiss for a few days but always eventually worked. Can you please help?