Is there any way you can have the secondary and third siren to sound without having to hold down a button for it to do it, al you do is press a button once to change it? I am not sure if there is any mods/plugins that do this but any help would really be appreciated!#

Thanks :) 

You'll need ELS V for that, which is still in development. There's a thread for it here:

 

 

And a video demonstration of the current development progress:

 

 

pipeline do you think we will get ELS for sure

12 minutes ago, tony said:

pipeline do you think we will get ELS for sure

It's in development right now. There's no doubt that we're getting it. The only question is "When?" and, to me, it is a very burning question. My impatience knows no bounds. 

You can use something like Xpadder. It's very powerful and lets you toggle keys, which is what you want, if I understand correctly :) At least that's what I use with my xbox360 controller, since holding the left thumb-thingy is a PITA :)

I'm planning on doing a tutorial for controller rebinding to better suit GTA5/LSPDFR, including the XPadder config, but I still need to find out some time to write all this down.