JLyons

If you mean the garage where you select your vehicle that is controlled through duty_selection.xml in the lspdfr folder.
It is suggested to create a copy of this file and place it in the custom folder. Leave the default one alone as a backup. More Info.
Note: You will have to rename the copied file to "duty_selection_eupfr.xml"
 
To add a vehicle open the XML file and look for
<VehicleDescriptions> 
 
</VehicleDescriptions>
 
In-between these tags add the following line.
 
<Description name="MENUNAME" fullName="FULLNAME" agencyRef="AGENCY">CARNAME</Description>
 
You will need to edit the AGENCY [Replace AGENCY with either lspd, lssd, sahp, fib, etc..]
You will need to edit the CARNAME [Replace CARNAME with the vehicle you would like to add to the garage.]
Not Required: You can edit the MENUNAME to whatever you want.  [This names the car in the menu]
Not Required: You can edit the FULLNAME to whatever you want.  [This names the car on screen while selecting]