Our micra did the same, the resistor pack need to be renewed. I can't remember if i did it myself but if you but the resistor (like i did) or what ever nissan call it in the book just get it fitted. I think it just plugs in, the dealers spare dept will know what your on about. I think the resistor pack was about around if not less than 10 uk pounds but that was 8 years ago. you need a nissan spares department not a high street car spares shop.
found this and lots more on google...
"Micky,
behind the glove box you will find the heater motor, mounted in the air box trunking. (To the n/s of the car.)
Close to this is the heater motor resistor pack, which is retained by, I think!, two screws. Remove this, and you will see a flat plate with a printed circuit board. Check this for visable damage. If you can see a "blown" track, then the resistor has failed"
Regards graeme.