When you replace a screen phone and the new known screen is very dim, and using any known good screen assembly with known good light sensor does not solve it, then you probably damaged something on the board related to back-light.
And it's usually any combination of the back-light filters, diode, or back-light driver.
- The damage can occur from shorting something on the board by working with battery and/or power connected, specially if using special tools.
- Can also happen due to pry damage, specially if using metal tools to unplug cables.
- Finally, can happen if the replacement screen has an uncovered back-light connection on the screen's flat cable (if you can see 1 thin cable connected to the main screen cable by 3 apparent solder points, then they are uncovered and you need to cover them before installing the new screen - or simply buy a quality screen where they are already covered). These uncovered solder points can short the back-light circuit and cause this same issue.
All the above is usually easily repairable with the right microsoldering tools.