aleex96

Not sure I fully understand what exactly you're looking to do, however to make vehicles choose-able from the garage, all you need to do is add the vehicle to the loadout of an agency in the agency.xml
 
 
If it is an existing agency, it will already be selectible. If it is a custom agency, you need to edit the duty_selection.xml
 
 
Now, your vehicle will be selectable, however it's name will be the same as the model name. To give it a proper name, simply add it to the <VehicleDescriptions></VehicleDescriptions> section of the duty_selection.xml. A simple copy/paste/edit job of one of the existing entries will do the trick.

This actually gets a little harder to explain, the deeper down the rabbit hole you go. Much of this can be done with the use of the custom folder, however im just going to describe straight edits.
 
You can modify existing agencies/create new ones by editing the Agency.xml
 
 
Each Agency needs one or more loadouts. These are where you specify the Clothing and Vehicles. (Check LSPD entry for how to set up EUP type models)
 
 
Alright, so at this point we have our custom agency, but we want it to spawn ingame.
 
In the regions.xml, we can define a custom set of zones for our new agency. (LSPDFR wiki will direct you to what each zone encompases)
 
 
Next, we need to edit the backup.xml This is where we will define our custom agency to spawn in our custom region.
 
 
We only need to worry about Local Patrol to have the units spawn, however if your custom region does not overlap with an existing region (Which would only happen if you remove zones from existing regions), you may wish to define the custom region in the rest of the entries in the backup.xml, so the game will send the correct back-up when requested to the region.
 
 
I know it looks a little messy, just give a shout about any particular area that needs a little more explanation.