BJT NPN Switch not fully turning off under certain conditions
Yes the SPI connection has a ground connection which is annotated on your diagram, is this what you're referring to? Also, keep in mind that you'll never get your transistor down to 0 Vce so your 5V supply is now only 4.5V or less depending on the current.

If you're using an isolated DC-DC supply (to prevent ground loop) for your 12V to 5V supply, you're defeating the purpose by switching the low side of the isolated Arduino supply using your radio's signal. For this reason you might consider switching the DC-DC supply side instead of the output side.

You can also switch high-side if you think the Arduino grounding is causing an issue but it adds an additional PNP. Basically use the 12V on/off signal from the radio to drive an NPN common emitter which can drive a PNP high side transistor for your 12V supply. The ultimate goal is to limit the galvanic connection between your audio/Arduino and car to only the AGND and signal wires (no ground loop!).







simulate this circuit &ndash; Schematic created using CircuitLab