Does anyone know where the FBIs carvariation file located on OPENIV I want to change some settings but I can't find it 

original file is crypted you can't change anything or look into. But you can overide that file by editing dlc carvariation located in two rpf for your choice mpheists and mpchristmas2, last one is way better as it's weight is around 60mb when in a other hand mpheists is about 2 Gb

\update\x64\dlcpacks\mpchristmas2\dlc.rpf\common\data\carvariations.meta

all you need to do is to create new section with a same name as your car bellow <variationData> but  above <Item> as it's start line for another car. And edit some values such as liveries and colors  or siren settings. Bellow i'll explain how does it works

    <Item>
      <modelName>yourcarname</modelName>
      <colors> <!-- colors section start--> 
        <Item> <!-- first color subsection start, numeric values below is a colors of your vehicle (primary, secondary, don't know, rims)--> 
          <indices content="char_array"> 
            0 
            0 
            20 
            156 
          </indices>
          <liveries> <!-- one of the livery slot assigned to color you have 8 liveries only and you must pick only one and set "true"-->
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
          </liveries>
        </Item> <!-- end of first color subsection--> 
        <Item> <!-- second color subsection start, same as above-->
          <indices content="char_array">
            113 
            116 
            20 
            156 
          </indices>
          <liveries>
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
            <Item value="false" />
          </liveries>
        </Item> <!-- end of second color subsection--> 
      </colors> <!-- colors section end-->
      <kits>
        <Item>159_slamvan_modkit</Item> <!-- tuning parts--> 
      </kits>
      <windowsWithExposedEdges />
      <plateProbabilities><!-- inside section is chasnces to get specific license plate style--> 
        <Probabilities>
          <Item>
            <Name>Standard White</Name>
            <Value value="25" />
          </Item>
          <Item>
            <Name>Blue Plate</Name>
            <Value value="50" />
          </Item>
          <Item>
            <Name>White Plate 2</Name>
            <Value value="25" />
          </Item>
        </Probabilities>
      </plateProbabilities>
      <lightSettings value="2" /><!-- light settingsfor specific car, police\sheriff use 1--> 
      <sirenSettings value="0" /><!-- lightbar pattern, sheriff use 13--> 
    </Item>