Hi Niki, yup, I follow you. My only error was that I 'assumed' you has the original cable supplied with the usa-spec because of the picture in your port # 308. My fault.
How long are your wires from the HU to the usa-spec? The shorter they are, the less coupling will occur. If you can seperate the i-bus/12v/gnd from the audio-L-R-gnd, that will help as well. Simply twisting the audio ground wire around the two audio lines (L-R) may be sufficient - approx 1 twist per inch is ideal.
I hope this helps. I don't think there is any reason for you to purchase an interface at this point. You have it working, you just have to clean it up a little bit.