Arivera

It’s relatively easy really, go into lspdfr in the gta v directory and go into data and find agency.xml, scroll down and find fib or LSSD(whichever agency you want to add to) keep note fbi agency doesn’t ambiently spawn, lspd will and LSSD use these if you want too see the vehicles driving around and as backup. 
 
Where is says <vehicle>model</vehicle> Just copy this line right underneath and set it as police old. Should look like this:
 
 
<vehicles>
            <vehicle>FBI</vehicle
            <vehicle>policeold1</vehicle>
            <vehicle>policeold2</vehicle>
</vehicle>
 
 
 
 
Also so if you have EUP go into custom inside the data folder and change the same line in the agency_eup xml.
 
if your adding to an agency backup.xml will not need to be changed.
 
this should ambiently spawn the vehicles around the map and allow them as backup, aslong as the agency ambiently spawns anyway. I.e lspd & lssd
 
 
edit: you can add <flag>SpawnsAmbiently</flag> 
 
To the flags if You want fib to spawn like police
 
 
 
Callam.