Bad alternator or neutral safety switch?
 

I have a 2002 Infiniti i35. The car died on me earlier today and all the symptoms seem to be a bad alternator. When the car was hooked up to another car via jumper cables, it would run fine, but when we took the positive cable off of the battery, the voltage quickly went down according to the multimeter. The battery read 12 volts with no jumper cables before trying to start. When we would try to start it you could hear the clicking from the starter. It was towed to our mechanic and they are saying it's not the alternator, but the neutral safety switch that is causing it. They also said the part if about 240 dollars, but online, it's only about 50. Would a neutral safety switch cause this? The car has always had a no-start while in park sometimes, then would always start in neutral.

Based on your very general description of what sounds to be a somewhat complex issue, it sounds like you have a couple, maybe three issues.

Car dying, interpreted as the car was running and then unexpectedly shut down. (Battery or alternator or both).

A battery showing 12.0 volts is inadequately charged. (Battery or alternator or both).

No no start in P, start in N. (Neutral safety switch).

Sounds like Ike a good amount of additional diagnostics or more details associated with testing already done is needed.