Cobber

The alarm systems on these cars (in fact all cars in my experience) seem to be more trouble than they're worth.
About the only thing they're good for is draining the battery!
I asked my auto electrician mate about this on your behalf, and as I expected he said that: He'd yet to see a car alarm that actually works as it should for longer than the warranty period and often less than that!
Alas, without seeing the car, he couldn't say what the problem might be...... other than if you have no problems with the sensor disconnected then that's the best solution.
He said that even if you replace the sensor (providing you can source one) it may not fix the problem.