Macbook Air screen fading randomly between off and full brightness

My Macbook Air (2015 model, High Sierra) screen is dead most of the time but randomly fades in and out. Sometimes it fades rapidly in and out non-stop for a prolonged period, before fading out entirely. The backlight is not dead - it works but is impossible to control. Please see this video for a demo of the issue. Does anyone know the cause, or solution?