The low-pass filter didn’t do the job. There must be a coupling issue between the CD player and the car audio. With the current CD player, I can crank the volume all the way up and just barely hear it on the car audio, if the signal from the RF box is cut. This definitely points to a coupling problem.
One thought I had was to build a Cmoy style amp, using the car +12volts as the power or this Simple Car Preamp. This should provide plenty of power, amplification and the coupling needed. There isn’t much to the circuit, so it could be built inside the car radio or into a car lighter adapter.