maybe you have damaged any display capacitor or backlight filter while disconnecting lcd from connectors.backlight ways if you have multimeter and know basics of board repair then check picture above and see if get continuity on marked points.
its a common problem in iphone 6 plus. your touch ic is causing that problem. see this video from ipad rehab and try if you can fix it https://www.youtube.com/watch?v=To6dJYgd...