The clicking noise you hear is because the voltage at the starter is dropping when you turn the key.
.
.
EG
Turn to Crank, Starter relay switches on and Energises the starter solenoid.
Starter solenoid pulls the starter gear into contact with the flywheel ring gear
(That's the click).
The Second function of the starter solenoid is to activate a high current contactor that energises the starter motor when the gear is in place.
What's happening to you is that when the starter is energised, the voltage across the solenoid is dropping, causing it to disengage the gear and the contactor.
Of course once the contactor is open again, the voltage at the solenoid rises, and so it closes again.
The result is a rapid clicking as the starter gesr is pulled in and out repeatedly.
The usual cause is a flat battery, or a dead one
(where a cell or two is short)
The other cause is a loose connection either at the Battery+ or the B+ bolt on the starter.
Also loose connections from battery- to Chassis, or Chassis to Gearbox.
Even 0.1 Ohms resistance at 100Amps results in a 10v drop out of 12!
If it's the Battery, the Battery+ or the Battery-Chassis
-ve connection, the interior light will go out or flash when you try to start.
If it's a loose connection at the starter or Chassis-Gearbox, the interior light will not be affected.
Final word,
If you end up replacing the battery
-
GET IT FROM TOYOTA.
Not only will it be the correct size
/ way round
But in my experience, it will be noticeably cheaper and have a better guarentee than the likes of Halfords!!
