Headlights/TailLights/MarkerLights

[57rookie]

So, I dont ever drive my truck in the dark so I never noticed before but I started it up in the garage in the dark to show my daughter my new Thrush Rattler exhaust I installed today and I turned on the lights.
I noticed right away that my marker lights, headlights, tail lights, and dash gauge lights were going from bright to slightly dim and bright again , basically flickering.
What would be the cause of this???????
Thinking alternator but my battery is solid and the alternator keeps it charged very well and it sits for weeks between start ups.
Thoughts????