Had this issue with my previous cheap stereo, and always thought the issue was because it was cheap. Went out and bought an alpine stereo for 500 bucks and I still get this god awfully annoying hissing sound. It doesn't matter what source, I can always hear it. Is there an issue with the factory pioneer amplifier?

Here is a video showing the hiss getting louder as I turn up the volume. The microphone doesn't capture it very well, and it is way louder in person.
https://www.youtube.com/shorts/w-_QNUZSAG4

 

if you have a spare piece of wire try using one side to touch the outer RCA and the other to touch the metal case of the HU. see if that fixes it. if not then look at a ground loop isolator.

 

Do you mean the RCA plugs? I don't currently have any RCA plugged in to the head unit, just purely the FL FR RL RR that I assume go to the stock amp.

 

hmm. thats odd because the stock amp gets low level signal from the factory so if you are using the speaker outputs on the HU you would be providing it amplified inputs. Generally the expensive harnesses have RCA connectors for those speaker wires to give the stock amp low level inputs.

Hows the volume? is it loud with minimal volume #

you could try something like this and connect the speakers to your RCA instead. if the volume is somewhat normal though then maybe dont try it
https://a.co/d/gin8uB5