I've had a problem with the audio cables from the HU to the amp picking up
interference from the engine and the heater fan etc.

It's definately not the power cable to the amp and I have got a good earth
on the amp.

The interference is not amplified by the amp it just represents as high
pitch whine which gets louder when the fan speed increases. Mostly it is
drown out by the sound levels of the music, but is annoying when listening
to soft quiet music!

If the audio cables are picking up this EM interference how best to get it
cured?

Can I use ferrite discs? are these any good?

Regards

Ian