It’s working for me, the chassis Parts and all the sub models are all welding together.
You don’t have any HIngeConstraints joining the Wheels to the chassis parts, or welds between the other models so that’s why they are dropping off. It looks like there’s a script that’s supposed to set up your HingeConstraints after the vehicle is together.
If you look in your Output window you’ll see 3 errors when testing. There’s something wrong to do with line 35 of your VehicleSeat2Module where it’s failing to find “parent handle” part. I didn’t get into it but I’d start with the naming of your parts.
So that probably means you need to look at the script that is driving your vehicle.
When testing are ANY of the Parts of the model still Anchored?
When you sit in the Vehicleseat in test mode are you getting the steering and throttle inputs in the VehicleSeat Properties?
Does the script you are using even use the VehicleSeat inputs or does it use UserInputService?
Did you follow all the directions in the script precisely? Many free model scripts are written with notes (or have a Read Me script) telling you to exactly where to put items like wheels and the scripts in different areas (folders, other models, or inside other Parts) of the model.