I always thought that a fluctuation of the indoor lights was a faulty alternator?
Now I did not publish this information but here is some help.
To perform the headlight test, start your car and turn on your headlights. With the vehicle in park, press on the accelerator while a second person observes the headlights. If the headlights flicker, dim or get brighter when the accelerator is pressed down, your alternator may be bad. No change in headlight brightness indicates that your alternator is likely fine.
Here is the source: How to Tell if You Have a Faulty Alternator - CarsDirect