My 1999 Landcruiser car alarm has started going off for no reason.

Okay, that didn't work. Any ideas on chasing down the problem?

Yes a factory system. Where do I find such diagram?

Toyota Factory service manual , or Chiltons/Haynes manual , OR also , for a small fee-'alldatadiy.com'. There is no 'instant' fix , unless you get a REALLY lucky guess! Knowledge of what you are doing/attempting to do , is helpful.Good luck.

This is from a Toyota forum on LC I removed the port installed alarm of my 97 last year. I had a few issues that weren't alarm related that popped up, but the alarm removal itself is pretty basic. Pull the sill plate and remove the alarm brain and harness from under the seat. Follow the harness and remove several of the connectors, and plug the stock harness back into itself. There are a couple of T taps you'll have to remove aswell. But you *shouldn't* need to do any cutting. 99% of the alarm harness is spliced into the factory wiring with connectors, so it's pretty tough to mess up. This is just going off of memory, but I did make a few posts about this process last year. This along with a repair guide might help.. If the alarm doesn't disable starter and it's just the horn that's the concern.. Remove horn relay and drive on!

Yes

maybe hood sensor. to try and verify/or not that the hood sensor switch is defective - disconect the wires going to the switch and see if you get random false alarms when parked. No false alarms = defectve switch. It may be worth a try

Great. Where is the hood sensor?

not sure on yours,just look for a sensor where the hood comes down to close,and makes the switch,should have wires running to it.there is a alarm horn,probably next to the brake reservoir,on firewall.there is also a horn relay in with the other relays,marked SEC HORN RELAY.

...........Google 'Toyota Magic Button'........

That's what I did and came up with that forum post!