Backlight not working - what are the usual causes?

momefarley, just like any other phone that had water damage,your phone will have to be cleaned properly. You did not tell us how you cleaned it. Disassemble your iphone with the instructions on here Take your time and familiarize yourself with the instructions first. Read the comments and the notes and make sure you understand the instructions. Once you have it disassembled take a look at the attached image and identify the coil and the area around the coil. Use some sterile water and rinse the parts off to remove the scale. Then clean it with +90% isopropyl alcohol and a soft brush.. This will displace the water and stop corrosion. Use this guide to get it cleaned. It is written for a 3G but applies to your phone as well. It is okay to submerge a board in isopropyl alcohol. After that, air dry all the components. Do not forget to clean the connectors on the board as well as the cables. Then re-assemble with a new battery This is not optional but a must to avoid failure later on. Check your board for any missing or damaged component. Re-evaluate after you clean the phone. If your backlight should still not work, test the 6R8 coil. Use this video or this one and see if it is time to change that coil. Now, if a coil replacement does not resolve your issue, there is still the possibility that it is your backlight IC 109B that is giving you the trouble. That will be harder to replace and will require special tools and techniques. Of course, you would also want to try a new display assembly. Water damage is the toughest to trouble shoot and to repair. Hope this helps, good luck.