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
Some Bosch-type relays have an internal diode between terminals 85 and 86 see 'S2' and 'S6' here but it's strongly advised not to use these as the terminals need to be connected the right way round to match your car's polarity. Get them wrong and you will blow the fuse, and maybe the relay.
Fitting the relay behind the dash avoids the effects of weather by the horns themselves. Behind the radiator is also usually safe enough, but to fit it without cutting wires you need to remove the purple/black wires from the off-side horn and run a wire from there back to the relay coil, then run another wire from the relay contact forwards to the off-side horn, and daisy-chain it from there to the nearside horn, removing the existing purple/black. You also need to pick up a purple for the other relay contact although that is easy enough from the fusebox, and an earth for the other side of the relay coil, possibly from its mounting point. Quite a lot of wiring and connectors.
If behind the dash you only need to interrupt the purple black between the horn push and the first horn, and pickup a purple and earth as before. A convenient place to do that without cutting wires is at the brush contact under the cowl on 1970 to 76 inclusive. Take a new wire from the horn brush to a relay coil terminal e.g. 86, and extend the original purple/black wire to one of the relay contact terminals e.g. 86. Connect an earth e.g. from the relay mounting point to the other contact terminal (30), and provide a 12v supply from a convenient purple (fused, always live) circuit to the other relay coil terminal (75):
However the cowl is pretty snug around the harness and column, and unless you relieve the inside of the cowl carefully to make space the two halves of the cowl may not come together cleanly, and even if they do they will probably be pressing the wires against the column which could eventually chafe through. Not dangerous - it would just sound the horn! For that reason I decided to leave mine as I did it originally i.e. cutting the purple/black behind the dash and using a 2-way terminal block to cross-connect the wires with the relay wires.