Do you have spark IV and know how to use it? Or are you just wanting to know the file path?
Well, I can help you with both, But Ill help you with the end part.
EFLC is kinda weird with the directory locations, You have the Core game files in spark iv first, Then, TBOGT, And TLAD.
Since your modding the FBI, You want to scroll down past a long list of text (htmls) Until you can see cdimages, open that up, Look around, Are all the regular GTA IV vehicles there? If your not sure, In the filter type fbi, And see if it pops up, if it does, your in the right one, If you mod this, it will mod it for both TLAD and TBOGT, If you want to acsess the Other vehicles from tbogt, you will have to navigate to those directorys.
For the first part, Here's a vid just in case:
Sorry for my post being a bit confusing, Just woke up, ill explain a bit clearer if you need me to.