So once you are in the .ytd, look for a thing called something similar to that or has a picture of the livery, copy the name of that to the png file and then drag and drop it into the .ytd. Here is a tutorial of how to do so, hope it helps
Go to the .ytd file, open it it and find the one called something along the lines of: Livry_1 or Sign_1. Then once you found those drag the png into the .ytd (make sure they are named the same thing leaving the .png at the end so it stays as a png file)
I have the same problem with the lights, I put in all the all blue files and textures but m lights with els are still red and blue. Is it something is the actual XML that I have to change? Nvm, I figured it out
Based off of what I have found listening and comparing sirens, I think they use a whelen cencom sapphire with a touch master delta rumbler
Here is a link to one