Sony 6CD aux-in distortion help?

Can anybody shed any light on how I might be able to get rid of distortion on playing any external source through the Aux-in socket connected to my 58 plate Mondeo Titanium X Sony 6CD system? (Its the MP3 playing version not the DAB one though and it has the Convers+ cluster).

I know the Aux is turned on, I've tried ipods, phones, even my old 80s walkman, used 3.5 to 3.5mm jacks, ipod to Aux cables, all different volumes, asked nicely, etc... and am at my wits end..

Even tried taking out the BT/VAC module unit and re-connecting but nada

Any suggestions greatly received
Thanks