Aux audio lead interference - RCD 500

[KernowGTI]

Hoping that someone can recommend a good earth point behind the stereo I can use for my aux cable / charger that's connected to the cd changer port on the back of the RCD 500 head unit. Currently I'm getting a speed related whine which I'm assuming is due to the lead (which comes with an earth) not being well connected.

Thanks

Apologies if this is in the wrong section