After reading your response, follow this step by step guide:
1- Attach the earth wire to earth point.
Check out this photo and see where the black ground wires are attached. This is driver side.
You have similar earth point on passenger side. Attach your earth to that point.
If you still have buzzing sound at passenger door, then
2- Phone / mp3 to phono RCA cable ---> amp. If buzzing sound disappears, certainly issue is with your wiring b.w HU and amp. Review that.
3- You can follow Technics guide to route harness if you are able to pass the wires through the centre.
Personally it is a lot easier if you pass it from the passenger side. I have run it on my passenger side and zero noise issues.
But mind you, in my second version of harness I have replaced the wires to RCA cable. It protects the signals as RCA cable is shielded and hence zero noise issues.
And please bear in mind that USA passenger side is Uk driver side where all the power cable runs.
So on that point, you are safe.
I reckon your passenger side cable is picking up the intereference from something.