Because the power for your headlights is pulled from your battery. When the dashboard lights dim after turning on the headlights the dashboard lights aren't getting as much power as they were with the headlights off.
With the motor running or off?
The headlights draw a lot of current, but should not visibly affect other systems.
There could be a problem with a worn out battery, an alternator that isn't charging correctly, or you have a bad earth between the battery and the chassis.
Check the large cable connection between the battery (usually negative these days) and the bodywork, or between the engine block and the bodywork.
For the 1999 Chevy Tahoe, automatic headlights are turned on by default. Turning them off requires opening the bezel on the dashboard and removing the light sensor.
test
the light bulb on you dashboard means you have and exterior light out if it isn't you headlights blinkers....ect its probably just a short
On most vehicles there is no such light. Turn on your headlights and look see if one is out.
Try rolling the dimmer dial upwards , just right of the headlight toggle, just short of turning on your interior dome light. Otherwise it's a short somewhere. jr
The button on your dashboard
light sensor
Before checking fuses, check that operator error hasn't turned off panel lights at the dimmer. Check owners manual.
What causes the dashboard light to stay on after your car has been turned off? 1994 Buick Century.
If you turn up your interior dashboard light settings, then the radio display will be brighter too. The radio dims to the setting for the dashboard when the lights are turned on.
That is not a button, but a light detector. When it gets dark enough, the headlights automatically come on.
There is an over-current condition in the tail light circuit. It could be a short to ground or a bad bulb.