Replaced starter, car started for 4 days. Fifth day car wouldn't start, just made a clicking noise. What could it be now?

test the battery

I would check your ground from the battery to the frame/engine and also the positive battery lead where it connects to the starter/relay

Check the spark plug. My truck did the same thing and it was because the spark plug had completely been removed.

I would go with the corroded cables. You would have power to everything even with the alternator out of the truck.

Anti-theft system has nothing to do with it . Battery cables , maybe , Park neutral safety switch possibly ! With anti-theft system problem you wouldn't a click or anything , that is if the anti-theft disables fuel or the starter . Plus the SECURITY light would be on if it were a anti-theft system problem . Your best bet , take it to a qualified repair shop that knows how to do starter circuit testing >
Starter Voltage Drop
How to voltage drop test starter motor circuit

And what's this identity anti theft system ? No such thing ! The vehicle has a content theft an vehicle theft deterrent system !
This system disables fuel not cranking of the engine. The VCM/PCM will not disable the fuel due to any BCM message for the remainder of the ignition cycle.
If the VCM/PCM does not receive the same password from the BCM as the last learned one, the vehicle will start and then stalls due to the Fuel Lockout.

may be a poor contact.
_________
MD802

tune up,fuel filter,air filter.

The alternator is bad

is the solenoid on the starter or is it in the engine compartment. if this goes bad all you Will get is click.click.click........ replace the solenoid and you should fire up with now problems

Probalibly the starter solinod.