@Belial_Lucifer
Use this mod as a base for your merged add-on pack (or alternatively see 'Useful to know:' below). Once it is installed (as per instructions on the download page you will have to add the 'dlclist.xml' line '<Item>dlcpacks:\addoncars</Item>' manually) the basic procedure is to add your vehicles '.yft' & 'ytd's to the 'vehicles.rpf' here:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\x64\vehicles.rpf
& their modifications/upgrades to the 'vehiclemods.rpf' here:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\x64\vehiclemods.rpf
Note: you add the independent '.yft' modification files ('pantera_wing1.yft' for example) into the 'vehiclemods.rpf' folder so it looks like this:
Don't put the vehicles 'vehicle-name-here_mods.rpf' archives into there or it will not work. Just the '.yft' files that are contained within any *_mods.rpf you have so that you will have the '.yft' modification files of multiple vehicles all in that same 'vehiclemods.rpf'.
From there it's just a matter of adding your vehicle's data (carcols, carvariation, handling, vehicles.meta etc) to the files contained within the 'data' folder here:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\data
The process is the same as what you do when you add a replace vehicle to the game. You copy & paste the block of data (being careful to maintain the correct syntax) from inside your add-on vehicle's data files to inside the files contained within the folder structure above.
Note: For the first vehicle you add to the pack you can just overwrite the blank meta files with the ones from your add-on but after that you have to add the data to within the files manually as you do with a replace.
content.xml:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\content.xml
'content.xml' tells the game what dlc files to load. You may have to edit this file (depending on how complex your add-on vehicles are & what files are contained within their dlc.rpf's) so it's worth familiarising yourself with how it works.
Say for instance your add-ons contain a 'vehiclelayouts.meta' & you want to add this to the 'addoncars' dlc you would edit the addoncars 'content.xml' to look like this:
<?xml version="1.0" encoding="UTF-8"?>
<CDataFileMgr__ContentsOfDataFileXml>
<disabledFiles />
<includedXmlFiles />
<includedDataFiles />
<dataFiles>
<Item>
<filename>dlc_addoncars:/data/vehicles.meta</filename>
<fileType>VEHICLE_METADATA_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/data/carcols.meta</filename>
<fileType>CARCOLS_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/data/vehiclelayouts.meta</filename>
<fileType>VEHICLE_LAYOUTS_FILE</fileType>
<overlay value="false" />
<disabled value="true" />
<persistent value="false" />
</Item>
<Item>
<filename>dlc_addoncars:/data/carvariations.meta</filename>
<fileType>VEHICLE_VARIATION_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/data/dlctext.meta</filename>
<fileType>TEXTFILE_METAFILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/data/handling.meta</filename>
<fileType>HANDLING_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/%PLATFORM%/vehicles.rpf</filename>
<fileType>RPF_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/%PLATFORM%/vehiclemods.rpf</filename>
<fileType>RPF_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
<Item>
<filename>dlc_addoncars:/data/carraddonContentUnlocks.meta</filename>
<fileType>CONTENT_UNLOCKING_META_FILE</fileType>
<locked value="true"/>
<disabled value="true"/>
<persistent value="true"/>
<overlay value="true"/>
</Item>
</dataFiles>
<contentChangeSets>
<Item>
<changeSetName>addoncars_AUTOGEN</changeSetName>
<filesToDisable />
<filesToEnable>
<Item>dlc_addoncars:/data/handling.meta</Item>
<Item>dlc_addoncars:/data/vehicles.meta</Item>
<Item>dlc_addoncars:/data/vehiclelayouts.meta</Item>
<Item>dlc_addoncars:/data/carcols.meta</Item>
<Item>dlc_addoncars:/data/carvariations.meta</Item>
<Item>dlc_addoncars:/data/dlctext.meta</Item>
<Item>dlc_addoncars:/data/carraddonContentUnlocks.meta</Item>
<Item>dlc_addoncars:/%PLATFORM%/vehicles.rpf</Item>
<Item>dlc_addoncars:/%PLATFORM%/vehiclemods.rpf</Item>
</filesToEnable>
<txdToLoad />
<txdToUnload />
<residentResources />
<unregisterResources />
</Item>
</contentChangeSets>
<patchFiles />
</CDataFileMgr__ContentsOfDataFileXml>
& then add your 'vehiclelayouts.meta' here:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\data\vehiclelayouts.meta
It's reasonably simple to do. For the most part you just add the new lines in the right place. Compare 'content.xml' files to get an idea of how it works & then copy & paste the relevant data to the correct place in your addoncars 'content.xml'.
Note: Different dlc's can have different folder structures so the main thing to look out for here when copying & pasting from one 'content.xml' to another is that the folder structure within 'content.xml' matches the actual location of the file in your merged add-on folder structure.
Example:
A 'content.xml' line like this:
<Item>dlc_addoncars:/data/vehiclelayouts.meta</Item>
would mean that your 'vehiclelayouts.meta' should be here:
\mods\update\x64\dlcpacks\addoncars\dlc.rpf\data\vehiclelayouts.meta
After that, any new 'vehiclelayouts.meta' data for more cars would be added within that same 'vehiclelayouts.meta'.
Tip: 7zip can be used to extract files from a .oiv installer for examination before installing the .oiv. Install 7zip & then it is as simple as right-clicking the .oiv & selecting 'extract here/extract to ...' etc
Extract the downloaded 'AddonCarsOIV.oiv' using 7zip & examine (or alternatively install the 'AddonCarsOIV.oiv' & then use OpenIV to examine) 'content.xml' & compare it to what I have pasted above to give you an idea of how adding a new file to the dlc works.
Tip: Sites like Diff Checker can be used to examine & highlight the difference between the data contained within 2 files or separate blocks of data.
Useful to know: If you find your add-ons are very complex (ie lots of different files & file locations) & you are having trouble sorting out your 'content.xml' & getting it to work correctly then it may be useful to know that you can use any dlc as a add-on merge pack (watch out for dlc file size limits though).
What I would do in this situation would be to choose a relatively complex add-on that I have (so that the 'content.xml' is already written correctly for all the possible necessary files for a wide spread of different vehicles. Say using a pre-made merged add-on pack & then removing it's vehicles yft's & ytd's & the vehicle data in it's vehicle meta files) & then add my vehicles & their data, using the process outlined above, to that dlc instead. That way you are just adding data to pre-existing files with no need to edit 'content.xml'.
. As long as you take it slow (install one mod at a time & make sure it is working before moving on) & make backups of everything you shouldn't encounter many problems that you can't fix yourself 
& dying having never got to play the game modded.
