Hi following from my "What colour are my horns" , I thought for any one interested I would post some photos of the inside of a Rover P5 3 litre horns and also show how I get them cleaned up and working. As you would imagine after many years they can get very rusty ( rust is a speciality of mine) They can stop working because the plunger, which is activated by an electro magnet, rusts in place due to inactivity. If you free this up you may get the horns working again. In my case I have been able to free up the plunger and clean it. The horns work 'Champion now'.
To get to the plunger you need to pull the rubber block from the two input terminals, this allows you to undo the two nuts that hold the points assembly together. Remove the metal spring plate; the Bakelite insulator; lift off the electro magnet contact and the right side input terminal. Now lift off the plastic points spring plate; the spring metal top-point and the left contact terminal. Then lift off the last insulating block. This leaves the plunger exposed. Carefully pull the plunger out of the electro magnet and examine. You can also see what shape the points are in. Having cleaned up everything replace all in the reverse order. Be careful when dissembling and reassembling as there are two insulating tubes surrounding the pins, these can be damaged and need to remain in good shape. Test the horn, they are very loud and scarred of our local flock of crows.