I recently installed my subs from my old car into my VYII Berlina using a LOC (high/Low converter) and they run fine. However, I just installed a 4ch amp for my door speakers and used the high-input port. Now I get a high pitched noise with ignition on and then alternator noise with the car on. So a constant noise with the key in ignition, and a varying noise with the car on getting louder with the revs.

I have a 4awg cable from the battery feeding along the drivers side of the car and have a 6awg ground wire grounded at a seatbelt bolt. I ran my + speaker wire from the headunit but picked up in the door. Specifically from the loom that goes from the body to the door accessible when you open the door and pulling out the rubber grommet. I did this for all doors rather from the headunit as I thought it would be easier to pick up from the doors than taking apart the dash to get behind to the wires.

The drivers side wires are running next to the power wire which I thought might be causing the problem so I'm going to change the speaker wire to full shielded cable and move them as far away as I can (further towards the center so running under the drivers side seats). I changed the grounding wire to a thicker gauge and found a better grounding spot which made a noticeable reduction in the noise however it is still quite present.

I have currently made my amp only on the LPF because having it on normal or HPF I get the noise. The LPF gets rid of the noise, but also most of my volume and mid ranges, etc. Just doesn't sound right, but until I can fix it, it's better than having the noise. I'm thinking about swapping to using another LOC instead of the High-input to see if that makes a difference.

Is there anything else I can do to get rid of the noise?? It's annoying as hell considering I spent all this money and time to get it to sound awesome and it's worse than it was. Sorry about the essay but thanks to those who read it and can offer help.