Ok, I fixed my horn yesterday in my S1 GTR, it has an S2 steering wheel, so no dice on the airbag, but!
As for the horn, there should be 4 wires going to the steering wheel, the green wire should be the horn active, you can check this with a multimeter/test lamp, test from the chassis to the green wire and you should have 12V there, then while still connected to chassis and the green wire (either with clamps, or a mate) pull the horn fuse in the engine bay, this should take away the 12V, confirming that it's the right circuit.
After that you can re -insert the fuse and connect the green wire to the single wire attached to the horn mechanism, this should pull in the horn relay (in engine bay) and sound the horn.
I hope this helps as I trolled the 3 threads about this on SAU and various others without answers. The methodology for find the wire is solid, so if the wire colours differ, testing them to ground and removing the horn fuse should determine which one is correct.