2-wire Horns (pre-79)
However there is conflicting information as to when this arrangement came into use. The Parts Catalogue shows the same horns i.e. with two terminals being used for the whole of rubber bumper production including 1977-80. The Workshop Manual diagram for 1977 models shows a similar arrangement to before albeit with the switched earth now via the horn push on the indicator stalk, and a 12v return path as before. The diagram for 1979 shows 12v being switched by the horn push on the indicator stalk, and a local earth return at the horns, i.e. only one horn terminal. Suppliers variously list the use of the later single-terminal horn from 1975, chassis number 410001 (June 76 and the start of the 77 model year) or chassis number 417001 (Sept/Oct 76). Clausager states that from the start of the 1979 model year in May/June 78 at chassis number 471001 (roadster) and 471036 (GT) "horns with earth return instead of an insulated return" i.e. single terminal horns picking up an earth from their physical mounting.
2-wire Horn Relay
Interrupt the purple black between the horn push and the first horn, connect that from the horn push to one side of the relay winding, and the other side of the relay winding to a purple circuit. Connect the other purple/black to one side of the relay contacts, and an earth to the other side of the contact. Given a lack of access to the purple black anywhere other than behind the dash or right by the horns, fitting the relay behind the dash avoids the effects of weather by the horns themselves.