Since I have messed with all of them except the stockades, what I did is I switched the fbi slot back to the original car in GTA IV (I have an entire backup of my original GTA IV unmodded). I switched all the car models in the .ini file to FBI as shown below:
The test trials were made once I got setup for my ped model and car ELS settings. I started the callout by typing "startcallout backup" in the panel.
Trial 1
The first trial was a City Police call for backup, I responded and when I rolled up on scene.... I found that it spawned my "Police" vehicle and not the FBI that was written in the .ini as shown below along with another cut from my LCPDFR.log, when I rolled up the callout also shutdown due to an error:
Trial 2
Note: I didn't reloadscripts afterwards.
I thought then maybe it was the car I was driving so I switched to the original fbi car and tried again. I typed "startcallout backup" into the panel again and responded like normal. What I found I thought to be quite interesting. When I arrived I didn't get an error, the person that was pulled over was there. However, the cop car was there but no cop and the lights were not activated as shown below: