Swapped logic boards to fix backlight problem, both boards bad now

A faulty lcd screen and/or lvds cable can short the backlight circuit all the way back to the backlight fuse, sometimes killing a few components along the way. Proper troubleshooting would require a known good screen and known good lcd cable and at least a multimeter and a very thin soldering iron to replace the fuses in case they have blown. What might have happened here is that you have two faulty screens that kill components on the logic board. You might be in luck since it can happen a faulty screen kills the backlight fuse only, leaving untouched other components.