This is going to be lenghty, tedious and ultimately rewarding process that will allow you to spawn any addon vehicle ingame in your story or singleplayer modes.
It does not rely on adding mods/mod folders, dlc lists in fivem folder nor game update/mod folders. It revolves around preparing your rpk and updating adequate elements of the original GTA5 files (save before editing).
Files being edited:
x64a.rpf
x64w.rpf
common.rpf
These files contain handling cfg, popgroups and ultimately dclist.
The initial part of preparing your package is by following this particular video: https://www.youtube.com/watch?v=FQxS59-ZnPY
Once you’ve encrypted every damn file in your “prepared” package you will create matching folder in x64w file (which contains ingame originated dlcpacks) and import packaged/encrypted file, common and x64a packages host your handling meta and popupgroups whilst x64w hosts dlc package files (this is where you put your addon car packages).
once your edits are complete run all 3 files through encryption fix and start the game
I currently run a TrainerV for SP but after adding popupgroups, loaded vehicles via dlc pack, and have addon vehicles spawned i can safely remove the trainer that prevents story mode from running.
Build 2699
Mods applied:
Manual transmission
RealisticDrivingV_2.5
TrainerV
Menyoo (keeps crashing if i try to spawn vehicle with adhesive + 31D but everything else works and TrainerV can spawn vehicles without crashing)
plugins.rar (5.5 MB)
plugins folder attached
Enjoy