I've observed a lot of ground loop problems in cars with a smartphone connected to a stereo line input and also to a USB charger. Some of the USB supply return currents pass through the shield of the audio cable and nastiness ensues.

Yeah have had this too.

Still stick with the 3.5mm jack over Bluetooth for quality.

Cars are one of the worst environments for analog electronics due to massive interference from spark plugs (basically wide band RF emitters), along with bad power from the alternator and high currents through the chassis ground.