Inspired by the knife-toaster thread, Mom always said not to turn the interior roof lamp in the car while in motion. We’d want to turn it on to read or play, but Mom said a cop would pull us over. Something about the light causing to many reflections on the windshield and making it unsafe to drive at night.
Is it truly illegal?
Not sure if illegal, but it does screw up your night vision and does cause glare/reflection on the windshield, so is not the best practice.
Earlier thread on the same topic.
I’ll be interested in the legality of this, but I suspect it’s one of those things that varies from state to state.
I have heard many times it’s illegal in Illinois, but the rationale given was that it was too distracting for the drivers of other cars.
A few years ago, I was having some work done on my car and was driving a loaner car. I parked the car at a restaurant in daylight, but it was night when I came out. I couldn’t get the inside dome light to go off as long as the headlights were on. I tried everything I could think of, and decided it was safer to drive with all lights on than with none. I was kind of hoping a cop would stop me because maybe she or he could figure out how the heck to simultaneously have the headlights on and the dome light off. maybe it’s because I’m so used to driving without the interior lights on, but I found it very distracting.
I pointed out the problem when I picked up my own car a day or two later. Even as I left the repair shop, a couple of their workers were trying everything they could think of to make the lights behave and opining on WTF was wrong.
I was going to post that link, but our payroll guy came over to give me some good news and I was beaten to the post. So since my answer was taken…
Does she also say it’s illegal to drive barefoot? 
I’ll stand by my post in the other thread:
You’d be amazed at what parents tell there kids just to get them to shut-up.
However it is illegal to ride a motorcycle barefoot in some states.
Hence, the big grin. 
In the 80s I was in a band that went to gigs in an old school bus that had been customized for the purpose of hauling a band and the equipment. The owner of the bus, who was also the owner of the band, would leave the interior lights on at night to help him not get drowsy. On one occasion he was pulled over and given a verbal warning for driving with the dome lights on.
If it’s illegal in any jurisdiction, surely someone ought to be able to link to an official cite.
The only mention I found in the current Illinois Rules of the Road booklet was (at the top of p. 22) “If drivers are being stopped [by law enforcement officers] at night, it is acceptable for them to turn on the interior light of the vehicle.” Which tantalizingly implies, but does not come out and say, that it’s not acceptable for the interior light to have been on before this.
Night vision is so easily screwed up that you have virtually none in most driving conditions at night. Advertising displays, street lights, and other cars’ headlights will all degrade it. The only time it really makes a difference is when you’re out in the country away from all those things.
As far as glare in the windshield, the angle that windshields are at causes the light hitting them to reflect downward. You’ll get glare and reflection from side windows, though, and that may be what you’re thinking of.
The real problem with the dome light being on is reflection in the rear view mirror.
Chicago (Illinois) driver here. I have a LED Blue dome light, only ever used it when stopped at a light or parked, but I do have Blue LED lamps illuminated the driver and passenger floors because i’m a douche and think it looks cool. Anyhow the floor lamps/light strips are brighter than the dome light and they are on all the time. Standing outside the car, I can see light in the window from a distance. I’ve had many, many many cops behind me in traffic with this on, and in their SUV’s it’s pretty obvious and bright. I have not been stopped. not even once. It’s a double whammy too, since cops probably don’t like anything except their own with blue lights. I’ve always heard of this too, about driving with any other lights except headlights as being illegal, but nothing has gotten me stopped.
There doesn’t need to be a specific law that says “thou shalt not drive with the interior lights on”. Most (all?) jurisdictions will have laws against careless driving or negligent driving expressed in fairly general terms, and they’ll apply to any situation in which you are driving while subject to an inappropriate and unnecessary safety hazard.
Just FYI in California it is illegal to have blue exterior lights anywhere on your car, including on your license plate. It gets you a ticket. And yes, that is how I know. I thought it was cool looking too. It wasn’t $100 cool though.
Just FYI, blue LEDs on your car are never cool looking 
Back in the 90s, in the UK, it suddenly became cool for truck drivers to have a string of blue LEDs around the windscreen (think Christmas tree lights).
A driver where I worked had some fitted and we told him that they were actually a signal for gay drivers to make contact with other gay drivers. They went straight in the bin, and I noticed that the fad died away quite soon after.
I’m not sure that implication follows. Most people don’t drive with their dome lights on, and most people are not pulled over. The quote may just be an assurance that the act of turning on the dome light should not result in unfortunate bodycam footage being shared on social media. Also, as you say, if driving at night with the dome light on is illegal, then the law should be findable.
(emphasis mine)
With the increasing presence of DVD players whose screens can be viewed by the driver behind the car, I would submit that this argument is no longer valid.
Depending upon the angle of the windshield, just enough glare means you can’t see unlit things you are approaching: pedestrians, animals, potholes, various detritus that hasn’t made it to it’s intended location (including sofas, mattresses, ladders, etc). All of which could cause an accident if you either hit them or don’t see them until the last minute & attempt to swerve.
It lights up your interior allowing other drivers to see what you’re doing. If you’re just driving it’s not much of a distraction.
IOW - STOP PICKING YOUR NOSE!
I didn’t see anything about it being illegal in the South Dakota Codified Laws.