I am sometimes a bit of "go big or go home" trouble shooter, and tend to just bench test the equipment with to 100% rule out other vehicle variables. Check the grounds, shorts, speaker; easy stupid stuff.... but failing that I like to start with a "no vehicle variable" bench test.
I know in both vehicles I have SOS sirens in I had to add noise filters to the siren side, I guess they are sensitive. The dealer had a bunch on hand so it wasn't just me. They tested 100% on the bench, then back in a vehicle same distortion. Not sure what that is, I am also no a SOS guru by any means. I can snap a pic of my setup tomorrow at work, it is in a Durango and a lot of Durangos went into service at the same time in my area which may be why the dealer knew the problem right away and had noise filters pre-wired and ready.