It's in the manual
On the Concert HU:
Hold down FM2 & RDS until the display shows 1000.
then use the 1-4 keys to enter the security code i.e. if the code is 2234, you press the 1 key twice, the 2 key twice the 3 key 3 times and the 4 key fur times.
Then hold down the RDS & FM2 buttons again until the radio comes on.
If it doesn't work then try it again with the ignition on
If you don't have the code then you need to get a dealer to do it for you, but presumably the repairers would have needed the code in order to test it.