The horn wire exits out the front of the steering box. If you look at the front of the box, there will be a small plate secured by 4 bolts. In the center of the plate is a hole through which the horn wire exits. The wire then connects to the chassis harness (should be a single black wire) which then goes to the 3 terminal horn relay on the left fender. The other horn relay terminals should have a red wire (power in) and a violet wire (power out to horns).
Series 1 Euro 1750 GTV
Series 2 USA 1750 GTV