I instead went in GTAV main directory/lspdfr/data and edited the ''agency.xml'' file, changing the vehicles they use. Here's an example.
<Agency>
<Name>Los Santos Police Department</Name>
<ShortName>LSPD</ShortName>
<ScriptName>lspd</ScriptName>
<Inventory>default</Inventory>
<TextureDictionary>web_lossantospolicedept</TextureDictionary>
<TextureName>web_lossantospolicedept</TextureName>
<Loadout>
<Name>Patrol Unit</Name>
<Vehicles>
<Vehicle>police</Vehicle>
<Vehicle>police5</Vehicle>
<Vehicle>police7</Vehicle>
</Vehicles>