Hi Trevor,
Many thanks for your message.
Connects2 have confirmed the interference is relating to a grounding issue; see their comments below..
Yes I have certainly come across this problem before. Usually on the fully amplified models and a select few platforms if that.
But where popping sounds and feedback noise is concerned, you are correct in assuming the grounding is the issue. However not from the wire that grounds the head unit. More so from the ground shielding of the RCA connections.
Therefore what you will require is the use of an in-line RCA ground loop isolator.
We do a version which is the IBGLI: http://www.connects2.co.uk/Product/ProductItem/IBGLI
Instead of going through the hassle of trial and error / the expense of ground loop isolators, I decided to swap for another harness that supports both high and low output of head unit to supply the audio. It appears the previous harness only supported low level output. This worked first time - http://www.connects2.co.uk/Product/ProductItem/CTSAD008.2
regards