Don't give up that quick! It's not dead. As you see it had 27 viewers in 1 day.

Maybe it's not easy to answer. As I understood you want do play music from the pc but need the tuner from the radio.
Connecting the pc with the amp is no problem. But extracting the tuner signal only is not possible unless disassembly of the radio unit and some soldering ...

I haven't taken out the radio, so I don't know exactly what input slots the unit has.

My idea for a solution:
Buy a FM interface for your pc and put the car antenna into it. Then just connect the pc directly with the amplifier. Should work IMO.
You can still keep the radio unit by adding some switches.

Is this near to your major plan?

Greets