Radio/cd/satnav/telephone, fiat head unit

[neon man]

Can any body advise on the repair or replacement of the head unit in my '03 Fiat Ulysse, everything works apart from the radio was told it could be a problem with the FM modulator? Any advice would be greatly received. The radio when switched on only has a FM hiss tried a different ariel still nothing. Fiat apparently need extortionate amount to replace the radio so do not want to go down that route.

Thanks

Tommy