If it is not because it was dirty, and is an "auto dimming" mirror, it may have started leaking the product inside that causes it to auto dim.
If this is the case, and that liquid goop falls on your dash or other areas of the truck, it could stain the/those areas..
I have not ever found anyone who has a better solution for an "auto dimming" mirror problem, than replacing it with a new one..
Do you know if this mirror is an "auto dimming" mirror ?
Dan F