The Nissan Path

So I've got a 90 Pathfinder with the 3.0 V6 in it, manual 5spd 4x4. It will not crank over or start. It just started acting like this last night, the starter relay clicks but nothing else happens. All fuses checked out, clutch pedal in, key turns fine, everything seems normal until I turn the key to START and nothing happens. I am pretty good with mechanics, and I'm officially stumped. For a while I was hearing a beep noise, not the door dinger, and I've never heard this sound in my life. Could it be some sort of anti theft system? Any ideas would be greatly appreciated!

Could very well be anti-theft. Have you tried disconnecting the battery and reconnecting? Somes a reboot helps.

Any aftermarket items that could contribute?

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Sounds like a dead battery to me. They /can/ just go bad like that. If you have a multimeter, check the battery voltage and what not.