Hope someone can help please!

The alarm keeps triggering on my B5 A4 avant- luckily just flashing indicators and no siren (think the battery in the siren is dead) because the car thinks that the boot is open when it is closed.

I changed the complete boot locking mechanism as was told that the closed sensor is in there, but the problem persists. In addition, the boot courtesy light no longer works.

Does anyone know what else could be causing this please? Is there another sensor some where??
Alternatively, how do I disable the alarm?

Any suggestions gratefully appreciated!!