You can get at the wires under the headlining, but not much help as there are no connections to test.
The heated window wire comes up one of the D pillars, I think the left side one from memory. I think there's a connection at the bottom, close to the rear lights but its almost a year since I was 'in' there.
Big brown with trace (I think green). Start by checking power there with the switch on. If no power there, then it's wiring / fuses.
If there is power there, check the screen. Do you get any volts anywhere on it when on? If it's all 12 volts, then the earth is the problem. If it's all 0v it's the power feed. You would need to check where the wires connect to the glass, which is at the top of the glass.
It may be a connector come off, a broken tab on the glass, or a wire broken as it comes through the grommet from the roof.