Sounds like the regulator in the alternator is going out.
flicker
A gas saving device for freeway speeds when running empty or light.
A flicker, a beam, a ray.
If all of the lights flicker I would check for a loose or corroded ground. If only 1 light flickers then check to see if the bulb is properly plugged in to its socket, or if there is dirt, corrosion or rust causing a poor connection.
only a light flicker or two.
Because there is a light inside of the warmer, and you see the reflection from the light.
The answer is flick or flicker! XD LOL
People leave their diesel trucks running because they think it is better. It is a truck with no spark plugs.
Flicker.
Possible alternator problem
If the lights flicker when the vehicle is running, the alternator is on it's way out.
You may have a pull on your batt, or a surge.