I have no idea what mod has a TURN YOUR VEHICLE OFF button in it but what has happened is that you have more than one mod which uses the Shift key or whichever key you are using.
What I do with mods is to install them one at a time and after testing LSPDFR after each mod is installed I then move to installing the next mod when I am happy the mod I just installed did not break something.
If you installed a mod and suddenly your siren no longer works, then you could go to the ini file for that mod you just installed and see if you can change the shift key in that mod to something else..... so there is no conflict.
When you press the shift key your computer is saying there are 2 mods that use the shift key while in the car....which mod do i use.
Hope that makes sense.
Also with each mod...eg the question about clothing....go to the page you installed the mod from...there is likely questions there from others saying there are no clothing for this customization...how do i fix it..... there are so many mods there is no way we can know which one you have installed.