I have seen some questions about this already but none of them seem to work. Can I make a UI mask so it will only show the right part of the car? Or should I use something else?
You could spawn your car off-screen and with a second camera record the car. Cast the game view of your second camera to a RenderTexture and use this RenderTexture in your UI, instead of the 3D model.
If you want to push it even further, you can then use your RenderTexture to create a Texture2D. This allows you to destroy the second camera and the car, and you can still use your newly created Texture.
I don’t know how much you know about RenderTextures . So here is a tutorial which I used a few years ago, when I had to learn about them and how to use them: How to make a Minimap in Unity - YouTube