This is the steering wheel of Alfa Romeo 156 2.0 JTS.

 

The headunit has 4 wires

1. RC-A
2. RC-B
3. GND
4. TEL/MUTE

Each button has resistance value like this.
RC-A:
Mute 390
Vol Up 170
Vol Down 70

 

RC-B:
SRC 391
Forward 171
Backward 71

 

Because left and right buttons have same value,

one resistor is needed to avoid duplication.
Put a 5k~10k ohm resistor between RC-A and RC-B.