Why is the touch screen glitching?

I have just purchased a faulty, used New Nintendo 3DS (so no chance of warranty repair), and I'm trying to figure out what to do with it. The touch screen is acting as if it's being pressed constantly, until I apply gentle pressure to the back left of the console. I have tried using a toothbrush to get any residue between the seams of the screen, and also loosened a few screws at the back, to no avail. I am wondering whether to do a full teardown and see if there is anything else I can loosen, also I have seen a method used on a 2DS with a similar issue which involves a small piece of electrical tape being applied to one side to alleviate pressure, would it help in this case? Failing the above, do I need to replace the digitiser? Thanks