I have adapted an old radio / cassette to accept a line in (aux) from my mobile / cell phone. Thread here: https://www.electronicspoint.com/threads/adding-line-in-to-an7465s-multiplexor.287201/#post-1759797 On connecting the phone to the stereo in the car and reproducing audio from it, there are two issues: 1) When the phone is connected via the 3.5mm stereo jack, the phone audio plays over the top of the radio audio, i.e. I am hearing both audio feeds through the speakers simultaneously. This issue can be "easily" solved however by plugging the charger into the phone. It seems that for some reason which I do not understand, when the phone has a power source, the audio signal is strong enough to "switch" the audio feed selected by the audio processor and silence the radio. 2) The problem with this is that upon connecting the phone charger I am getting a significant amount of engine speed related whine and crackle. I am not so bothered about the phone needing to be connected to the charger as I like to take the opportunity to charge the phone in the car, but I cannot live with the ground loop interference. I assumed that the noise was due to the fact that I had the phone charger plugged into the cigarette lighter, whereas the stereo is connected to a seperate dash loom, so I installed a second lighter socket which takes its 12V+ from the same switched 12v supply as the stereo and also grounded the new socket to the stereo earth (in the dash loom), but this has made very little improvement (some, but not enough). I then suspected a poor earth through the dash loom, so I ran a new wire from the common stereo / charger earth to the car body earth lug (nice and clear bare steel), but this made no difference. The audio ground of the aux cable is connected to the stereo ground, and I have considered disconncting this, leaving only the positive audio channel feeds connected, but on the test bench this gave a lower volume audio feed from the aux input so I am reluctant to do this, but would it potentially help? Short of installing a ground loop isolator (last resort), does anyone have any ideas of anything else to try?