You can still use the bal-unbal transformers suggested, exactly as shown above. They work with any level.
At low level, they would perhaps be best mounted in a metal box. There is no problem with 'mains hum' pickup in a car, but there will still be electrical noise from other components. Pickup can be reduced by using screened cable, which RCA connectors were designed for.
E90 325DSE manual - The model number on the boot may not be JUST the model number on the boot