Couldn't find a thread on how to mount horns specifically the DD B3's under radio of a road glide. Any advice will be appreciated

 

On a low power setting drill thru the fairing underneath the HU with a 1 11/16 hole saw. Some people put the drill in reverse. I didn't and it turned out fine. Just remember that the back of the tweeters are bigger than the hole you are drilling so make sure the tweeters are spaced accordingly. I did mine real close and it works but it doesn't leave much room to tighten them down.

 

Here you go for reference. I did mine with everything installed except the HU, just now noticing how off center they are.