If this is the same as the M reg which I have just done you dont use radio keys. There are four screws securing the trim around the radio/heating controls, near the ash tray and radio. Remove these and pull trim forward and unscrew the four screws securing the radio.You will need an adaptor wiring block to adapt the new radio to Nissan loom, but dont trust Halfords to supply the correct one, as the one they list for this only fits 2000+ models. It took 3 trips to convince them it was wrong!!!!!as was their computer!!Motorworld had the correct one. [£13.99].I also needed an aerial adaptor to suit the JVC unit I fitted{£3.99}.Refit trim as above.2 yrs after question was posted but hope it helps someone else.
Howard
August 2007