Galvanic Isolation in Automotive Environment
Get a 12V DC DC converter with isolation. Make sure the input voltage can handle 12V and the output voltage is a 12V output.

There are some that can be found at digikey

With some searching some can be found with terminals.

Before you try that, try a clamp-on ferrite on the wires to see if you can increase the inductance and stop high frequency noise.