It is very difficult to get to the alarm horn wiring from underneath the car, that's why the thieves bend open the bonnet.
However, the alarm horn will not stop a determined thief. If your car is in a vulnerable spot, they will break it open and take it no matter what bumper you have fitted. Trackers are meaningless, they will just rip open the dash to find it. Bottom line, don't leave your vehicle where it is vulnerable. If you have to, make peace with the fact that it could be stolen and make sure your insurance is paid up.