I just picked up a Radio Shack "Piezo Siren" part# 273-0079 and some male 9-pin connectors. I already have the HD security system on my SG so this was simply a plug and play affair...
The OEM plug in is in the left fairing cover plugged into a dummy harness.
Simple plug in the Radio Shack siren attach it with double sided tape and you're good to go.
Total money spent is a whopping $8.60...!
Warning, the instruction are trash. At lest for my model Harley. I had already watched a video on Youtube on how to install the siren on a 2008 and later touring model and it was different than the instructions that came in the box.