It's called a dimmer.
a dimmer
A "dimmer" switch will change the brightness of lights.
Often the switch on the dash that controls the brightness of the dashboard lights will also turn on the courtesy lights if you turn the brightness all the way up. Otherwise, refer to the owners manual. If you don't have an owners manual with your vehicle you can get one online or at your local dealer.
Be sure that the dash light brightness control switch is not turned off.
The dash light brightness control may be turned off, check the switch.
Either the brightness control switch is turned off or the circuit blew a fuse.
Check the light dimmer switch to see if the lights go dimmer or they stay at the same brightness. It sounds like the dimmer switch is either stuck or bad.
If the instrument lights come on but the brightness isn't working you probably have a damaged light switch. They usually aren't too expensive, but on the other hand it depends on the vehicle and the parts distributor. Also if the brightness is controlled by a switch other than the headlight switch, then this rheostat may be bad and need replacing.
Check the dome light override switch to be sure it is off. This is the same switch that controls the brightness of the dash illumination lights.
The dimmer switch near/on the headlight switch is a potentiometer and it controls the dash light intensity, as well as manually turns on the interior lights at the high(bright) end.
There is a switch for that. It is in the dash illumination brightness control switch.
Please check the fuse first. If the fuse is good then the rheostat switch is the next component to test. The rheostat is the dimmer switch you use to control the brightness of the dash lights. It will probably be part of the headlight switch. Best of luck.