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Note: There is a reply below this post that is just as helpful, make sure you read that!
 
There are so many car mods out there that it's frustrating when you can only have a certain amount in game (as replacements), and have to keep switching between all the ones you like or just want to try.  This text-based tutorial aims to teach you how to install cars as a add-on/standalone!  Please note, this tutorial does not cover add-on cars that have their own dlcpack.
 
Quick notes before you begin.
 
* You must use a mods folder!!! For information on how to use this, click here. (OpenIV website)
 
* Mpchristmas2 is just my preference for standalone cars.  Some people use other DLCs such as mpluxe.
 
* There is a standalone car limit.  If you exceed yours, download this gameconfig for unlimited add-ons.  NOTE:  I have not personally tested this one, back up your gameconfig before installing!!!
 
* All standalone cars must have a vehicles.meta and carvariations line!  Otherwise it will not work properly.
 
* Handling.meta, YMT edits, etc are all optional, you do not need to edit anything else aside from the vehicles.meta and carvariations for the car to work.
 
 
1) In OpenIV, navigate to mods-->update-->update-->common-->data-->levels-->gta 5, and extract "vehicles.meta" to your desktop.  I advise making a folder on your desktop named "original vehicle meta", or something similar so that way you can store these files and know exactly what they are.  You won't actually be editing this meta, but you will be taking individual meta lines from it to use for your add on cars. For example, if one car is a Dodge Charger, you'll use the police2 meta lines.  CVPI, police lines.  I don't think it truly matters in the long run which you use (aside from the handlingid needing to match the car) as long as they are similar and not completely different (IE using the meta for the Pounder)
 
2) Grab the "police" meta from this vehicle meta.  Either paste it in a notepad or just hold onto it.  Note:  If your vehicle is a non Law Enforcement vehicle, just take the meta of a similar type vehicle.
 
3) Navigate to mods-->update-->x64-->dlcpacks-->mpchristmas2-->dlc-->common-->data-->levels-->gta 5.  Extract this vehicles meta to your desktop.  Open it, and paste the meta line.  Change the modelName, txdName, and gameName to that of your add on car. 
 
4) In the vehicles.meta there is this line:  <audioNameHash />
 
Audionamehash dictates how your vehicle sounds.  If it remains <audioNameHash />, it will not have any sound.  You will change the audionamehash to whatever suits the vehicle you're installing.  So if you install a CVPI, it'll be police. There isn't a huge variety of namehashes for police vehicles.  There's no police Explorer, so you'll either end up with an Explorer that sounds like FBI2 or SHERIFF2. 
 
Note: The following only applies if you are using ELS!!!   I, however, have started using the namehashes of regular civilian vehicles.  In the case of the Explorer, I use gresley.  This means that your vehicle will sound more in tune with what you're driving.  However, this also means the in-car radio will work.  The only way to disable this is by turning the music all the way down in the settings.  As I said, there's a variety of what you can use, and I have those listed in another topic.
 
Save, and drag the edited file from your desktop back to OpenIV.
 
5) In OpenIV, press the back button twice so you see the "carvariations.meta" (mpchristmas2-->dlc-->common).  You will also need to extract and do the same thing in terms of naming it.  Change the name, save, and re-add to OpenIV.  I've included one of my carvariation lines you can use as a template for all your vehicles.  Do note a few things:  1) Some vehicles require custom carvar settings.  Most, however, will work fine with this.  2) For non police vehicles, change sirensetting to "0".  This setting is only for emergency cars.  3) <kits> deals with the mods that the car will have.  I, personally, never mod my cars, so I just ignore that. 
 
 
6) If you haven't already, download the vehicle files for the car you want, and rename them to match your add on car.
 
7 ) In OpenIV, you can either click the back button until you get back to the page with "common" and "x64" folders (mods-->update-->x64-->dlcpacks-->mpchristmas2-->dlc).  Once on this page, go to x64-->levels-->gta 5-->vehicles-->xmas2vehicles.rpf.  Click the "edit mode" button, and drag and drop the vehicle files from your desktop into OpenIV. 
 
8 ) Click "edit mode" again to turn off edit mode, and you're good to go.
 
Final notes:
 
* In the carvariations, sirensetting must be set to at least 1 for it to have siren.  There are other siren settings you can use.
 
 
* In vehicles meta, <flags> must have FLAG_LAW_ENFORCEMENT and FLAG_EMERGENCY_SERVICE