the hot screen loom is stand alone so what you need to do is first find the wires that go to the screen in the bottom corners and see if it's getting power when you turn the screen on

the screen only works with the engine running so you need to start the car before you poke testers into bits of it

assuming you have power there the only fault then is the actual screen itself

they normally have a 60/40 split so that the drivers side, even though it sees the same current, clears quicker as it's got less to clear

you will also have hot washer jets so you can unplug these to see if they are also working, but they tend to be very brittle and after a couple of years worth of hard work can lead to having broken internal wiring in them