Why is my horn and reversing camera not working?

You’re probably looking at two different, distinct problems here. It’s pretty unlikely that the car’s horn has anything to do with the reverse camera and vice-versa. The fact that the horn sometimes works and sometimes doesn’t suggests a wiring problem rather than anything wrong with the actual horn unit. You need to check the contacts along the entire horn circuit including the contacts on the horn itself as well as the contacts within the steering wheel that trigger the horn. In modern cars with air-bags, however, this is not a job for the casual tinkerer. Don’t forget the fuse-box, as the horn circuit will have a fuse and this could have shaken loose over time leading to the on-off problem.

The reverse camera problem could be down to the screen or the camera unit itself. Again, you need to check all the connections to make sure that everything is connected tightly and that no moisture has managed to get into the circuit (which is entirely possible as the tiny camera unit is mounted on the outside of the vehicle). And while it might sound silly, check that the lens of the camera isn’t covered in dirt or grease which could also leave you with no image on the screen. Clearly, when you select reverse and the screen goes blank, the camera system is trying to show you what’s behind you, so power is getting to part of the circuit. But either the camera isn’t picking up an image, or the screen is refusing to display it. An auto electrician sounds like a good one-stop shop for both your problems.