I beg to differ on this. this is the simplest way to wire a horn.
While you are absolutely correct in the suggestion of reversing the polarity, what this gentleman needs to do, in my opinion, is check the polarity of the wires coming out of the steering column and ground the earth(-ve) to any metal part of the searing column assembly.
Then he can connect the other wire to the metal ring. which would the complete the circuit.
He would also find mebbe 2 metal strips, which some amount of flexibility on the inside of the steering wheel, when he turns it over.
These in turn assist in completing the connection by rubbing against the metal ring.
The horn button would have an inherent grounding inside the body, where one wire goes to the switch/button and the other is ground thru' the metal of the assembly.
And if you say that there are 2 wires coming out of the steering wheel then one needs to be connected to earth, and the usual way of doing it is through a slot that would be provided in the wheel housing, wrapping it around a washer which fits over the 19mm diameter of the steering column and after placing this washer over the metal rod of the steering assembly, tightening the lock nut, thus providing the earth for the -ve terminal.
As far as the relay goes, it would most probably be in the engine compartment, placed close to the battery, so as to reduce internal impedance of the wires, and thus provide maximum current to the horn. But most times this relay is installed with twin horns and not the usual continental one bellow pieces usually fitted by car manufacturers, to save production cost.
And about the indicators coming on n off. the plastic housing where the metal ring sits, would have little pinholes on the underside where a 1/4 inch in length pin (usually supplied with the wheel or else can be removed from the original with a pair of pliers) fits in snugly. This pin brushes against the release mechanism of the indicators and they come back to inactive position from either side.
Care to be taken that the pin is inserted in the reciprocating hole, keeping the diameter of the original position in mind.
So in short;
Wire from the steering column connected to the metal ring.
Out of the two wires from the wheel, one to the steering hub with the washer, and the other to the metal strips rubbing down on the ring.
Yeah, now I guess this should do it.
Sorry for so many edits, as me memory fails me.