The mod provides a choice of interior cabin lighting: blue, green, orange, purple, red and white for all standard and some mod trucks.
The backlight is available in the “Driver’s plate” and “Navigator’s plate” sections. You can even mix them: assign “On with dimensions” to one plate and “On with flashing light” to another plate, or assign “On with dimensions” or “On with flashing light” to both.

On with dimensions: the backlight works when the parking lights (dimensions)
are turned on. On with the flashing light: the backlight turns on when the flashing lights are turned on.
The backlight is only visible when it is dark outside your truck.

Make sure that in the settings:

The “Light range of visibility” is not disabled.
The “Additional car headlights” checkbox is selected.

Update in preparation for patch 1.58

Credits:
Eddie Yantz