What's this other plug in the back of Monsoon?

[VeesGN]

My Monsoon radio came out of a '99 Z-28. It has the 4-plugs grouped together with a total of 20 pins. It also has another plug with 9 pins. Does anyone know what it is or if I'll need it?

 

[1970gsx]

Possibly for steering wheel control inputs?

 

[turbota]

Was the original car that the radio came out of equipped with On-Star ???????

 

[stss96]

Its the plug in for the factory cd changer.