I actually just found a fix myself. If you have something overwriting your dlclist.xml from within your MOD folder and you edit your raw dlclist.xml from the ingame files anything you try and edit will not actually be run since the game will be running off the overwritten one. Edit all of your cars into the MOD dlclist.xml and the issue should go. This is just one fix that is so obvious it flies over your head.

I've actually managed to find the fix myself.
A mod that didn't involve touching the update folders decided to overwrite the dlclist.xml. So i went into the mods folder and deleted it. Now every car is working.

I'm going to keep this post open though just incase anyone else with the issue finds it helpfull

CHECK YOUR MOD FOLDER

I'm having a big issue with add on cars not being able to spawn into the game.

I have 5 current add on cars that do work but I have about 7 or 8 that just wont load in, I've had history with installing cars in this game so i know most of the basic tricks, (Although that was two years ago)

My dlclist.xml is perfectly fine and the mod folders are all in their respected named file in the dlc folder But ingame I can't get them detected by the add on car spawner or Menyoo trainer.

Currently running latest GTA version and this is now going on for every add on that I am trying.

I would greatly welcome any assistance.