I think it's a "glitch" inside the interior lighting circuits/controller probably "enhanced" by a faulty/failing LED bulb.
I noticed again today that the drivers side map bulb was on again, dimly.
I suspect that has a fault and is doing something to the electronics.
Just opening a door turns on the map lights along with the interior lights so I'd guess that's done by "earthing" one side and that the other is always live (+12v). The separate switch will probably do the same.
It seems likely that the interior light circuitry "earths" those bulbs to a lesser extent all the time for some reason at present.
I'm going to replace the bulb that is currently dim and see if that sorts it out. They're just a bit of a PITA to get at. 
Edit: Just having another look it's actually intermittent, so maybe the bulb isn't seated properly.
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
Last edited by HPsauce; 23rd October 2017 at 08:28 PM.
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