It sounds to me like there's either a loose electrical connection or you've got a "smart" alternator that's not as clever as the manufacturer wanted it to be.
Things to look at: battery terminals, fusible links, main connection on other end of battery negative, earth point under ECU. Thinking about it, the entire issue might be due to a fault in the instrument cluster which Nissan wisely (cough cough) shared its earth point with the stereo. Sometimes that means when the stereo's earth fails, it tries to earth through the only other vehicle-body-connection which is the antenna, and if that's a little dirty, you'll get fantastic results in the dash as everything that needs an earth searches wildly around all the inputs for an earth point, so you'll get the weirdest set of lights showing - and the fix is to add a wire from the stereo chassis to any dash mounting bolt.
I'm not sure that's what your problem is - you might have a cracked fusible link, which might be hard to spot. Use a multimeter in continuity mode across the link (it's part of the connections on the positive battery terminal connector) and while measuring continuity, tap the centre of the link. If it's cracked (which happened to me) you'll find the multimeter will show the connection failing. If you don't have a multimeter you should be able to use a test lamp connected to the negative battery terminal and the side of the link away from the positive battery terminal.
If it's a "smart" alternator, it will have (somewhere, I saw it once and don't remember exactly) a green wire attached to it and removing this will render the alternator "dumb". The "smart" alternator was designed to stop taking motive power from the engine so that the car used (slightly) less fuel and felt more responsive, but this ended up causing issues with DC-DC chargers and more. If your alternator is getting stuck in "smart" mode, that green wire should fix it. You can tell: watch the voltage as you're driving. It should always stay above 13.5V, mine stays above 14 (it's allowed to get to 14.7V and the battery will be safe). If it falls below 13V, your alternator is NOT charging. That might just be a belt too, but I am assuming the mechanic has already checked the belt, and the pulley (if the key in the alternator shaft is missing or broken, the pulley can spin on the shaft = no charging).
