@Kana765 Basically how the mirrors are working in this game is, (as far as i know) there is something called in game Screen Space Reflection. So this reflection thingy has the entire game world as a low quality fake world. So the reflection you are seeing in the mirror or on the calm lakes&rivers is just an image of low quality fake world. It's not a real reflection.

The games that has real reflections are the ones have Ray Tracing implemented to the game. Again as far as I know and if I'm not mistaken, those games uses the real reflection so anything you can normally see is also visible on the mirror.

So anywho, if I may merely say my opinion so, don't bother with it much since many things will be invisible anyway. I ended up crashing so many times because of invisible objects while I was trying to park my vehicle from mirror. They honestly don't even show the back with a realistic FOV anyway. There should be a warning saying "Objects in the mirror are extremely so much closer than they look." I gave up trying to park my car from mirrors and now just switching to 3rd person view while parking. Which this feature is actually realistic even if it's not exactly the same Called 360 degree view.

For the black mirror problem, try using another vehicle that has good quality mirrors. The best one I can suggest is https://www.gta5-mods.com/vehicles/bmw-z4-add-on-lods-wheels . It has a rearview camera as a mirror (Can be toggled with Extra 1). Check both of how it's mirrors are and rearview camera mirror is;

If they look normal and you can see the game world on the mirrors, then the problem is with the car you installed. You can ask the author to make proper mirrors but don't have much hopes in this. You said though at it's YouTube video mirrors are working. So if they look black in Z4 mirrors as well, then it's one of the graphic mods you have installed (most likely). and you gonna need to do a troubleshooting at that stage. Basically rename your mods folder to modsX or whatever you like. Then create a new mods folder. Start with installing the car first and check are the mirrors working with vanilla game.

If yes, proceed with installing the graphical mods at small chunks. I have VisualV and it's not causing any problem for me. Don't know the others.
If no, then backup your d3d11.dll , d3dcomplier_46e.dll , dxgi.dll and delete them from game folder. Also rename scripts folder to scriptsX as well and try again.

Reflection Quality in game settings also effects this as well. Try setting it to Ultra to see if that's the problem.