The problem with feeding the aux in from a non-grounded device (such as a portable player) is that you will get lots of noise as it tries to track the floating ground.

If the noise goes away when you put a piece of wire between the aux in shield and car ground, you just need to do this in your install - we still have some mk1 grounding cables sitting in the despatch room which aren't doing anything useful - I suspect we could send a set if this would work for you.

Hugo