There are systems that cover the original rear view mirror. On ebay, it will be listed something like this:
7" TFT LCD Monitor Car Rear View System Backup Reverse Camera Night Vision Kit
They are as cheap as $49 for the monitor and camera, and work reasonably well. I used one for years in my auto. This model has the disadvantage of being very dark when used as a normal reflective mirror. It does work, and you can adapt to the dark mirror but it is not ideal.
There are other models that have a very small monitor made into the replacement rear view mirror. I've not used one of these, and think they are too small to be helpful.
Fred