If you converted your license plate lights to LED, but they don't seem very bright or cut out, then you need to fix the ground problem discussed in this old thread:
https://forum.ih8mud.com/40-55-series-tech/185982-78-fj40-license-plate-lamp-question.html
There's actually a flaw with the design. The license plate lights are fed only a power line (except for the last few model years where they added a ground wire). Ground is through the bracket to the door, then from the door to the car body. The contact between the door and the body can be very loose, thus the ground connection is either weak or intermittent.
The solution is simple: Add a wire from any bolt on the door (e.g. door latch) to either a bolt on the tub, or to the unused ground wire in the harness.
With a solid ground, your license plate light will stay bright.
https://forum.ih8mud.com/40-55-series-tech/185982-78-fj40-license-plate-lamp-question.html
There's actually a flaw with the design. The license plate lights are fed only a power line (except for the last few model years where they added a ground wire). Ground is through the bracket to the door, then from the door to the car body. The contact between the door and the body can be very loose, thus the ground connection is either weak or intermittent.
The solution is simple: Add a wire from any bolt on the door (e.g. door latch) to either a bolt on the tub, or to the unused ground wire in the harness.
With a solid ground, your license plate light will stay bright.