Again referring to 3.7 Roadster diagram the horn and headlight controls share the same dash connector C109. Would not be surprised if earlier Roadster is not similar - very likely the area where you have the problem. Good luck finding it
Richard
2018 Roadster 3.7 1966 Land Rover S2a 88 2024 Royal Enfield Guerrilla 450 1945 Guzzi Airone
