I have bought two loud Snail horns. Good wiring diagrams so easy to fit and wire.
Use a meter to check the already fitted wires for polarity and replace live and earth as far back toward the cab until good wiring is found. Solder and shrink over joint. The rest is easy using Lucas connectors with 12v wire following instructions.
I bought twin snails from eBay. £10 and 30 mins to fit.
Graham Davies, January 2017