Nexus 5 screen testing - how to boot without all parts connected?

I have a nexus 5 where I replaced the screen and the backlight functions on startup but the screen doesn't display anything. I tried plugging in the old screen (LCD damage) and it also has the same problem. I am wondering what might be causing the failure. Is it possible for this phone to boot when the case isn't fully assembled? Do the cameras and speaker/mic need to be connected or will it still startup when those are removed?

I need to find a way to test different possible problems and replacement parts (such as the flex cable) without needing to fully reassemble the phone each time.