Board Thread:Questions and answers/@comment-35306464-20200410131139/@comment-30049823-20200410220654

is an article about modding the NiteFinder and includes a nice pic of the internals.

I'm gonna bet $1 that your problem is that leaf switch that the trigger pinches together when you pull it partway. Could be corrosion from moisture, or it just wore out and snapped off.

But you'll almost certainly have to take it apart and connect power directly to different spots (bypass the battery box, then bypass the switch). If the LED lights up when you feed power straight to it, then you KNOW it's the battery box, switch, or wiring, and don't need a new LED.