If you have to turn the volume up all the way, then maybe the gain is set too low?
WA2EIO is on the right track.
Googled: "loud hiss car stereo"
https://www.audiocontrol.com/knowledge-base/hiss-coming-thru-speakers-installed-product/AudioControl always recommends you ground the unit to the battery via the negative terminal. That option isn’t always available, so the next best way to ground it, is to the chassis of the vehicle. When you do this, you want to make sure it is grounded to that actually steel chassis, a lot of newer vehicles chassis are aluminum, and if you grounded to aluminum, that can cause problems. When you ground to the chassis, you also want to make sure there is no paint on the chassis. Paint can interfere with getting a solid ground connection.
You also want to try to have all of the aftermarket equipment grounded to the same point.
Another good step to confirm if it is indeed a grounding issue, is to take a temp wire and run it to the negative terminal of the battery. Using jumper cables is a good option, since you can just clamp the jumper to the terminal. You would then gather up all of your grounds to that wire. If the hiss goes away, then its a grounding issue and you need to re-configure your grounds.
If we’ve done all of that and you STILL have the hiss, then we would want to check your gain staging.
Meaning, the signal going into the converter may not be high enough, so you need to turn up the gain more, which will then bring up the noise floor more.
HTH...
WX7Q
Jim