I mean, 3D print the final part. If it works for fucking rockets going into space, it can work for cars too.

3D printing doesn't "work" for rockets in general. 3D printing works for specific rocket components (engine cooling channels) which are difficult to manufacture using traditional techniques like machining or casting. I can't think of any components like this on cars.

Relativity Space did try to 3D print an entire rocket (including the tanks, LOL), but cancelled it after the first launch, switching to a design where only specific components will be 3D printed.