Looking at this wiring Diagram (
http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1971_Part-2.jpg)
They show relay #49 wired as follows:
Terminal 30 - to Fuse panel and terminal 86 (Red Wire) - This is the power to the relay coil and the horns.
Terminal 85 - Horn Button (brown/white wire) - this ground the coil via the horn button.
Terminal 86 - Jumpered to terminal 30 (red wire) - this provides hte power from the battery to the horns themselves.
Terminal 87 - To Horns (Yellow/Black wire) - This provides power to the horns when the coil is energized.
Terminal 87a - Not used