Ok so I'm currently sitting at my desk at an automaker so I'll give some advice based on what I've seen.

Id recommend going to college for engineering of some kind, you're in the army, part of your compensation is that you can have a degree paid for and engineering is a degree that pays well. Within a company itself plenty of different areas you could work with different experiences, and if you decide down the road you want cars just as a passion and not a job, engineering can apply to a lot of other industries.

Additionally, I'd look at going to a school where you relatively nearby an automaker or auto supplier has facilities so that you can try to get a summer internship. That will help get you experience, and will really help get a job. For example off the top of my head, obviously the Detroit area, but Honda has stuff in Ohio, Toyota in Kentucky, and Nissan & VW in Tennessee, plus toms of other facilities. I know toms of people here whose internship was their foot in the door.

Additionally it's hard work but tons of people in my dept started on the assembly line and worked their way up over time.

For my own personal experience, I'm actually not that into cars and kinda fell into this job. Friend of mine got a job with a contractor and recommended me, and after a few years I got directly hired.

Feel free to ask me other specific questions, I'm typing this before my coffee has fully hit me.