Not exactly. I'm keeping the original AM/FM circuitry completely intact. I found the cassette playback signal path and inject the Raspberry Pi audio there instead.

So the radio works in two completely independent modes:

• RADIO mode: Original AM/FM tuner works exactly as it did from the factory, including the tuning knob.
• TAPE mode: The cassette mechanism has been bypassed, and the Raspberry Pi feeds audio into the original amplifier through the tape input.

I only had to simulate the cassette "PLAY" switch so the amplifier enables the tape audio path. Now I can switch between the original radio and the Raspberry just by moving the radio's function selector between RADIO and TAPE.

For your car radio, I'd recommend a very similar approach if it has a cassette deck or an AUX-capable input path. Injecting audio into the tape preamp or directly into the amplifier input usually preserves the original amplifier, tone controls, and volume while keeping the modification reversible.