Rear lcd gets whiter and whiter or has horizontal lines.

I just got a faulty 5D mk2.
When I connect a good working rear panel (I also have a good 5D mk2), the lcd screen starts to get whiter and whiter (it doesn’t get brighter, just more white) so I have to turn it off in about 3 seconds.
I tried powering on about 3 times and each time the rear lcd either got whiter or had horizontal lines in different places.
It looked like it would destroy the lcd if I kept it powered on and the lcd that came with the camera only lights up all black (on both cameras) so it is broken.
But my good lcd still works perfectly on my good mk2.