Nissan Micra radio code
The solution here is easy. As you already have the serial no. of the radio/cassette - now all you need is the VIN (chassis) no of the vehicle it came from which you will find on the registration document.
The reason for this is that on original equipment stereos such as this the unit is totally locked into the Nissan network not only by the serial number of the unit but also the vehicle which it came from. This is an anti-theft measure which tightens security on the code.
On OE stereos like this there is no other way to retrieve the code because of the addition of the chassis no. into the equation.
You must go to your nearest Nissan dealer and they will run the numbers for you and give you the code straightaway.
The good news here is that Dealers will do this for free or a very small nominal fee. Once you have this information to hand there is no labour involved for them in retrieving the serial number of the unit itself and they generally do not charge anything for supplying this service.
Hope this helps. Please do rate my answer and thanks for using FixYa.