Does anyone know where/how the boot sensor is? The other morning I was sleeping and the alarm on my car (99 TH Magna sedan) went off. Turned it off but about 30 minutes later it went off again. Popped the boot and closed again, all was fine.

Climbed into the boot later and had a look around but couldn't see a microswitch or anything that would be used to detect the boot opening.

Also every now and then I need to open the boot and shut it again, otherwise the car will not lock, so I suspect that there is a sensor problem there somewhere.

I'm pretty sure there's no sensor but it's something mechanical. For instance when you close your boot into the clip, that might touch some metal which tells the system it's closed. This is exactly like with the doors, well at least with the older cars. When you close a door, you push a sort of 'pin' in and it touches some metal, and when it's not touching metal, the interior light goes.

That's what I assumed, but can't see anything mechanical that it might be. As an aside, I have found the switch for the front doors, but can't see the switch for the back doors, but they work. assume they may be internal. But the boot is all external hinges etc...

Pull off the bootlid carpet (if you have it) and check the mechanism itself for corrosion etc; Also remove the catch and clean under it, ensure it is well grounded.