Sgt ECarnby

Unfortunately there is nothing that helps me.

BUT i have a solution.
 
carvariations.meta located in mpchristmas2
add this lines and it works.

<Item>
      <modelName>police3</modelName>
      <colors>
        <Item>
          <indices content="char_array">
            134
            134
            0  
            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>
      </colors>
      <kits />
      <windowsWithExposedEdges />
      <plateProbabilities>
        <Probabilities />
      </plateProbabilities>
      <lightSettings value="1" />
      <sirenSettings value="13" />
    </Item>

You didn't read properly! :O
Let me quote it for you! :)
 
Trust me, this guide helped me so much in understanding how car modding works, you should read it thoroughly! :)