The following image shows you with circuit schematic of the car radio muting of Nokia CARK-91, a car kit that offers
mute feature of the car radio automatically during a conversation. A muting unit or auxiliary relay must be used when the car radio doesn’t have a mute feature available as this feature provides for safer handsfree operation.
The muting system features that in standby mode the car radio works normally as the yellow wire of XCRM is not grounded but when receiving a call, the line is grounded then car radio is muted. You should use 200mA fuse to protect the XCRM output from short circuit.
Connect series with the car radio
main supply if considering relay use, and install the relay in the main
supply feed so it not to break the memory supply (ACC or +MEM). You can
have the manual kit and the installation guide of Car Radio Muting System in the Nokia CARK-91 and Circuit Schematic here.