It doesn't get any less complicated than this.
Something like this one could work: https://www.amazon.com/dp/B07PV8ZN1X
However it poses a few risks, it might not have diodes in it to prevent backfeeding to the radio, which is really bad if you have a constantly on 12v socket in your car, and it can also potentially damage the radio internally beyond what this mod can do.
I also don't recommend that cable specifically, I did a really quick Amazon search so I can't guarantee that it has proper protections in place.
Maybe low power? I suggest getting a y cable like this:
It's also possible it's because of how the ssd is formatted. You might have to install NTFS support on the pi to use it.