Alright, first thing first. You are working with non-ELS vehicles, which means they will have extra's such as in this case a spoiler, but their can be additional extras like light bars, etc. That doesn't mean you can't use it in the base game, it just means you have to fiddle around with a few different things.
The first thing you need to do is change the name of the vehicle in your .rpf to Sheriff (or whatever your wanting to change it too). Once you've done that do the same in your vehicle.meta, carvaration.meta and carcol.meta file. Now, as far as where they should go I have mine placed as follows:
dlcpacks -> modding -> dlc.rpf -> common -> data -> carcols.meta & carvariations.meta
dlcpacks -> modding -> dlc.rpf -> common -> data -> levels -> gta5 -> vehicles.meta
Also make sure you are referencing the load sequence in content.xml it's in the main directory, if you aren't you need to add these lines:
<Item>
<filename>dlc_modding:/common/data/carcols.meta</filename>
<fileType>CARCOLS_FILE</fileType>
<overlay value="false" />
<disabled value="true" />
<persistent value="false" />
</Item>
<Item>dlc_modding:/common/data/carcols.meta</Item>
After you've done all that load your game and see if it works. As for the spoiler, you can remove that manually with simple native trainer (or any trainer). If you want the car to load with preset extras your gonna have to go into the vehicle.meta and force load whatever extra's you want for example if you only wanted extra 1 and extra 2 to load you would do:
<extraIncludes><Item>EXTRA_1 EXTRA_2</Item></extraIncludes>
It's a lot to take in, I learned this all on my own and I swap back and forth between ELS and non-ELS cars, so if you need anymore help just ask.