I've got a 2005 Nissan Almera with a cd stuck in the cd player. I took it to Halford's, they couldn't replace it because the controls for the stereo also control the heater/air con. I've tried everything I can think of to get it out and failed. I rang the main dealer to ask how much it would cost to replace it; £600!
You have a number of choices you can pay the dealer prices you can go on ebay there are plenty of secondhand items available or you could buy a standard radio cd and a new aftermarket radio fascia again available very cheaply on ebay.
If you know about Nissan Almera N16 audio from this era, then I stand corrected. I am not familiar with the Almera
I have experience of the 2002 onwards Primera audio. They are NATS coded to the ECU but not by inputting a code. Used units have to be virginised by Clarion, Nissan or Audio shops with the right kit. If not, a Secure message will appear. Typically £40 upwards.
If it is the CY450, there is one on ebay for £50 with warranty that has been reset, so should plug in and pair itself with the ECU.
For 2003, Nissan has made a lot of detail improvements to its handsome Almera hatchbacks. And, since the launch was 300 miles away near the Sunderland factory, thinking of killing two spuggies with one clem, I borrowed an Almera Tino MPV to get me there and back.