Back light burnt out?

Devices left in the sun, especially when inside a car, can cause the LCD to overheat and crack. Like you, I would have thought that it is just a screen replacement. However, in this case I suspect that the cracking may have overloaded the backlight circuit somehow causing the backlight filter to blow, or worse case, the entire backlight circuit.

Plug in the phone and shine a bright flashlight on the screen. If you can see a dim image, then the backlight circuit is the issue and will require a micro-soldering repair.