@PlayingVoyage
Add-on I presume & by "spawn by model" you mean spawning it by name, where you type in the name of the car manually? If not, try that instead.
Also, run through & post here, step by step, the procedure you have used to install the car & I'll see if I can spot anything you might have missed. Thanks
A link to the car in question, so I can have a look at it, might help too