My father in-law is a hoarder, can’t hear and refuses to clear his car for safe driving view. He also refuses to get hearing aids. Any advice?

If you look you will see many vehicles on the road that have no clear glass in the back. As long as there are side mirrors and you can see out of the car and use the side mirrors that is legal. I think there has to be 2 side mirrors though.
Check your State Rules of the Road to be sure.
Is his apartment like his car and filled with "stuff"? Is the landlord aware? This may be a building or Health code violation.
The other thing I am wondering is...Does dad have dementia or is he a hoarder or both? When was the last time he had a physical and did he have any cognitive testing done.
One of the things my Husband started to do was "collect more stuff" he had always been one to save stuff and find stuff bit it got a bit more intense as I noticed/suspected the dementia more.

Certainly the DMV should know about his hearing problem. Yes, people with no hearing do drive, but usually the law requires enhancements on the vehicle, like extra side mirrors. If he has to take a hearing test to keep driving and his license is updated to show restrictions of wearing hearing aids, like there are for wearing glasses, that would be good, but I don’t see much hope in this situation because it looks like his mind is going. In which case you should be making plans for both of them to live in a place where they can be cared for.

Why not go into his car and clear out those visual obstructions yourself? If dad puts more junk in his car again to obstruct his view, I'd hide his keys or disable the car to prevent him from driving before he kills someone. Get him to the doctor to get tested for dementia too if possible because nobody without cognitive impairment would refuse to clear his field of vision while driving, let's face it. There's more going on here than simple obstinacy which is also a sign of dementia. Common sense has left the scene here.... you need to figure out why? Dementia is the first thing that comes to mind.

Good luck.

Definitely report him as an unsafe driver, maybe more than a few times. Sounds like an updated medical evaluation where the doctor knows the concerns of the family in advance is needed. But that can’t be forced, and unfortunately you may in the group here of “waiting for the fall” That’s waiting for an event that will force change, and it will come

kj1958, it is your father's choice not to get hearing aids. You and your Mom just talk in a normal voice, and if Dad can't hear you, so be it.

As for refusing to clear his car for safe driving, I am surprised that your Dad hasn't been stopped long before now by the police for not having a clear view. Dad can still drive after clearing his view even if he has no hearing aids. Deaf people do drive.

As for being a hoarder, that problem is a tough one to fix. What does your Mom think about it since she is living in that situation, I assume.