This is the best cheap DIY mod I have done to my 07 Ultra. It is to put an alarm siren that works with the factory alarm.
Go to Radio Shack and purchase Part #273-079 less than $6.00.
My local Radio Shack did not have the part in stock, but allowed me to pay for it in the store and shipped it to my house free of charge. I had it 2 days after I ordered it.
Don't know why HD sells the alarm and siren seperate, but read the post about the "Radio Shack" siren so I thought it would be an easy inexpensive fix and it was.
here is the post that I read about it in posted by ROLLNTIGER.
Removed the right side saddle bag and cover and then removed the electronic control module, 2 hex head screws. Removed the plastic cover, two nuts. Cut the head off the factory wiring harness. The factory wiring harness has three wires black, brown, and green. The siren only has a black and red. Take two pieces of shrink tubing and place over the red and black wires from the siren. Wired the red from the siren to the brown from the wireing harness and the siren black to the bike green. The black I just tucked back into the wireing cover. Test your connections! Slide the shrink tubing back down and used the blow dryer on high setting to shrink the tubing. The siren is small enough that I used some 3M emblem tape and secured the siren where the wiring harness was located. Simple using "cutacroshorty" as my instuction manual.
I was surprised at how small the siren is and never thought it would be loud enough. For those that have the alarm but no siren this is a great mod.
I did not cut the head off the factory plug. I stuck the wires in the holes in the factory plug and taped it all together. When I pulled off my plastic cover the plug was being held in a clip on that cover. As far as the sound, the speaker itself is very loud (120db), but after it is taped inside the plastic cover and the side cover is put back on it is not as loud. It is still very noticeable, but just not blaring loud. Any other questions let me know. I don't know if it will work on the 06 models but it seems like I read where there is a difference between the 05 and the 07, not sure about 06.
Good luck and feel free to PM with questions.