check the rating of the windscreen, get a relay (timed?) big enough plus 5 or 10 amps (always over engineer electrics) and some wire to suit, make up a small tail from an adequate source, personally i would use a known 80a supply i ran to the engine bay for accessories, if you have no such thing you could take a wire from the starter where the battery connects or the alternator or best bet run one from the battery box, fuse it almost immediately -
a wire of that size un-fused is called a firelighter.
run this up to your relay and then the switched side onto your windscreen. Ideally putting it in the battery box will let you have a short run, a fuse then the relay down in the battery box neatly mounted on the side, then just run a small wire for switching and a big wire to the screen up to the dash area, deck the screen locally to a good earth using an appropriate sized wire, deck the relay in the battery box and take an ignition switched live from up where you mount your switch. Take that ign live via your switch onto your relay switch wire so when you press it the relay activates and powers your screen.
Personally i would invest in a 10 minute relay and use use a push to make switch, hit it once and the relay will stay on for 10 minutes.
The above is perfectly adequate and safe way of doing it, if you prefer you can loom it all up to have both switch wires and l and n feed to the screen all loomed together back to the battery box.