Horn wiring

I probably should know this, but have had a mental block! I've removed
a sports steering wheel fitted to my car (suzuki swift) because was
missing the horn button in the centre. The previous owner had re-routed
the horn to a button on the dashboard. I want to refit the original
type steering wheel with the horn in the centre.

I have traced the power wire to a bundle of wires attached to the back
of the windscreen wiper/indicator circuit block located on the steering
column, from which it appeared to have been disconnected. When I touch
this wire to a copper plate within the circuit block, the horn sounds.
Question is - where should this wire be connected?

I can't see any obvious terminal from where it might have been
unsoldered and disconnected. From my recollection of auto electrical
theory, the wire would have to be connected to a point at which it
could only be earthed when the horn button was depressed thus
completing the circuit. This wire has to go somewhere. It has me
baffled. Can anyone help?