[tony_fury]

I did extensive testing with the radio ground wire when I replaced my CDR-220.

When the wire IS grounded, it reports a fault of the stereo being removed.
The wire is connected to a spring metal tab that when the radio is slid out, makes contact to ground and triggers the alarm.
When the factory radio is push in, it pushes the metal tab open breaking the circuit. Open means no alarm.
When the wire is grounded, this causes the symptom of beeps when arming your alarm. (Fault)

If you install an aftermarket stereo you need to remove the wire from the metal tab and tape it up. The result is an open circuit and no beeps when arming your car.

Bottom line:
Normally open = no fault
Closed or grounded = fault

Hope that helps.
T