If you have business tape (Becker be3350) as I do:
After you remove tape player find those 4 blue capacitors.
Desolder two ones on the right.
Solder capacitors back only to pinholes in the front
One capacitor is right channel and the other one is left channel.
Use ground from the casing or pcb.
Now you need to trick the radio into thinking cassete is inserted.
Cut this thin line with a knife (black arrow).
Thats it, put it back together and install it in the car. Tape mode is now aux.
Sound quality is excellent, there is no hum or noise.