If it's working intermittently from the steering wheel, chances are that your clock spring is broken.
Intermittent test: turn the steering wheel a little one way while pressing it. Turn it the other way. Any difference? Yes? Clock spring it is! If not - and it's because you bumped the brake or clutch pedal just right - it's the switch on either of those pedals. Same deal - while tapping each pedal around, press the steering wheel switch.
So, assuming it's the clock spring: you have several choices.
1) Buying from Nissan. You can do this, but it's best done if you're married and have at least one child you can sell to them.
2) Buying aftermarket. Good idea, have done this myself, but make sure you get one with an "R" stamped on the upper face for "Right Hand Drive". Otherwise, your indicators won't cancel properly. See below about centering too.
3) Buy just the ribbon and make it a complete DIY. You'll have fun taking it apart (it's fairly obvious, just fiddly) but when you reassemble it, lightly turn it so that you find the centre (count the turns in each direction and balance them out so that you're in the middle). Of course, you MUST have made sure that your front wheels are pointed straight ahead before you remove the steering wheel!