Sorry It took so long to put this up, I couldn't find my PDM diagram.
Not sure if this helps. The wiring diagram I have is for a “rev C” PDM. But it looks like the horn low amp “control circuit” comes from pin 1 on the PDM module (where the fuses are), then the diagram shows it just goes into the “Master Harness”.
I believe it should go into pin 3 of the HCI module 12 position connector (the HCI should have 3 connectors, a 12 pin and two 8 pin).
It looks like this connector has a gray wire going to the horn button on the wire harness side and nothing on the HCI side. I think this is where you should hook your short wire, in this “open pin” opposite of the gray wire.
The 2005 up hand controls operate on a grounding function, so if you attach your “long wire” to the gray wire (from the PDM) and the “short wire” to the gray wire (to the HCI) going to the horn button on the controls this should make the “control circuit” function and operate the horn relay.
The PDF i have is too big to upload so I took some screen shots of it. I will try to email it to you if your email is listed here.