You'll get reflections off both the front and back surface of the glass. This is called 'ghosting'. Because it doesn't all reflect off the coating intended for it, some of the light actually travels through the uncoated windshield and is lost. This is where it gets dim.
The coating is actually manufactured INTO the glass as it'* assembled. A glass company can put an optical coating on that area of your windshield, but I don't know what the cost is.