He's talking about the alarm horn itself.
The black is ground, the purple is +12v when key is on, and the tan is the triggering signal.
When the triggering signal goes to ground, the alarm sounds.
On the two wire horn, you have +12v and tan wire. When the tan wire goes to ground, the circuit is completed, and the horn sounds.
On the three wire, you have 12v and ground, and the innards of the horn chirp when 12v is first applied. That is just to let you know the horn works. Again, when the tan goes to ground, the alarm sounds.
So, just test it. Wire it up like I described, go back to the engine and ground the tan wire from the temp switch to see if it works.
The innards of the three wire horn may prevent it from working just with the purple and tan connected.
I have a two wire horn if you need one.