My dad is refusing to consider my mother's health and her desire to have the car in the garage. He is behaving very selfishly and only cares about having access to 'thing he might need'. I know it is hoarding, and hoped appealing to safety would help - it did not. Can I call the fire department and ask about concerns for fire hazards? He yells at anyone who challenges him - possible early dementia...
It seems to me you need to do what you can to reduce the physical risks to them both, even if it makes dad angry. I might feel a little differently about it if your mom's safety wasn't at risk. Dad's decisions effect her, too, and her advocate is you.
StandingAlone gave you some great advice. I'd at least try that approach. Giving your dad choices is important. His attitude could be the start of dementia, or it could be just that elders often get tired of being told what to do. But since he challenges all authority, dementia may be an issue.
For your current problem, if he refuses the alternate approach of his very own storage shed to the car can be in the garage, then maybe you should chat with the fire department. If they have a reason, they could be able to help by investigating and telling him he needs to move his stuff (to a shed?).
Good luck. I'm sorry to say that you've only just begun, so please keep coming back.
Carol