Hey there,
first what Vehicles are you trying to install? Simply replacing Models,editing them or add new ones?
If you want to use ELS you need special .xml files for the vehicles/models you want to use ELS whit. Those are normally included if you download a Vehicle from Modding Sites.
Since im assuming youre trying to replace Vehicles...
Quick guide:
If you simply want to replace a Vehicle you will need OpenIV
MAKE SURE TO BACK UP YOUR FILES AND/OR USE A MOD FOLDER
In OpenIV navigate to GTAV/update/x64/dlcpacks/(Latest patchday)patchday20ng/dlc.rpf/x64/levels/gta5/vehicles.rpf
There you will place your replacment Vehicle files,for example fbi.yft , fbi.ytd , fbi_hi.yft
Most Readme Files will tell you to use specific patchday Folders since theyre outdated but if you use the lates one it will be fine as it overwrites older ones.
As for ELS make sure the Vehicle is combatible whit ELS wich will often be stated by the Creator,if you use ELS Vehicles whitout it,it will result in broken/not working lights.
Open your GTA Main Folder(ill assume you already have ELS installed),there navigate into the ELS Folder and pack_default. That is where you want to put the coresponding ELS .xml Files.
Whit that done everything should work fine,atleast to my understanding. Take it whit a bit of Salt since im no Pro in modding but whit this ive ran good so far.
Vehicles that will be of interrest to you:
police - LSPD Cruiser
police2 - LSPD Cruiser
police3 - LSPD Cruiser
police4 - Unmarked Cruiser
policet - LSPD Transport
fbi - FIB Cruiser
fbi2 - FIB SUV
sheriff - BCSO Cruiser
sheriff2 - BSCO SUV
If you have a Replacment Vehicle for lets say the BCSO Cruiser but for whatever reason you want it as City Unit you can simply rename the file from "sheriff to police2"
Just always make sure theyre comatible and that it will only replace the Model,handling lines will still be vanilla