Grounding for car stereo adapter PCB
The noise is caused by ground loops.  Wikipedia has a nice article about them:

https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

A floating supply avoids creating a ground loop, but when you power your board from a supply referenced to audio ground, it creates a ground loop through the head unit wiring and power connector wiring.


  1) How applicable the star grounding is if there are multiple connectors (each with a separate ground wire), which may, or may not be referencing a "common" ground somewhere further.


It won't help since the ground loops that are causing problems are external to your board.


  2) Is it possible to improve noise with a different/proper grounding on my PCB, or should I isolate either the audio output or power input?


Isolating the audio output or the power input would solve the noise issues by eliminating the ground loop.  There are also several other ways to solve or minimize ground loop problems.

Here are a couple of ideas for your situation:


Bundle your power wiring together with your head unit wiring and connect your power GND to the chassis as close to the head unit as possible.  This will minimize the differences in magnetic fields that your power GND and audio GND are exposed to.
Assuming that the head unit wiring has a good GND and your board only takes a few milliamps (about 200mA or less), connect its power as follows:








simulate this circuit &ndash; Schematic created using CircuitLab

The values shown for R1, L1, and C1 are just a starting point and should be tailored to your board.  R1 can be included in L1 (DC resistance of L1).  R1 should be as large as possible without causing too much voltage drop or power dissipation, so its value depends on how much current your board requires.  Increasing C1 will decrease the audio noise coupled from the board's power.  Increasing L1 will decrease noise coupled from the car's +12V and from the board's power, but larger inductors cost more for a given current rating. 

If you go this route, check to be sure that at least one of the following conditions are met:


The head connector's GND makes a connection before the signal lines
+12 is never connected when the head unit is being connected or disconnected
Neither the head unit or your board would be damaged by +/- 12V on the signal lines