For the benefit of others, the PIKO part number is 36131.
Interior Lighting for 3 Axle Coaches Buy modeltrains from PIKO | PIKO Webshop ✓ Top quality ✓ Fast shipping ✓ Excellent support
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You need at least 2 metal wheels as well, the middle one is not used as a pickup so you could leave it as plastic.
Take care trying to remove the roof, it doesn't just pull off, you need to undo some screws hidden behind the door surround at both ends, see picture, then it just lifts off. My later model has two screws each end, the older models with set back doors have one central screw at each end, but the lighting kit is the same for both.
The pickups are small brass strips that push against the metal wheel axle. They make contact with a brass pin underneath that passes through to the interior of the carriage. From there you have some push on cables that join those pins to the lighting strip with the LEDs mounted on it. No soldering is necessary.
Here are the instructions.
It's designed to support analogue DC or DCC. You can fit a small PIKO decoder to the lighting bar that goes across the top of the carriage but I wasn't able to determine which one. I used an old LGB 55022 decoder which is fairly useless as a motor decoder but works fine for carriage lighting. The brown and white leads are track power from the lighting bar, the yellow and green go back to the lighting bar for the overhead lights and the other two go to some rear of train red indicator lights that I added.
The lighting kit is quite expensive for what it is. The current RRP is €54.50 and a pair of 35mm wheels #36165 are €33.50. (You used to be able to buy sets of 3 metal wheels but only in pairs now). If you have some basic DIY electrical and soldering skills you could make your own lighting kit at a fraction of the price.
The latest PIKO video shows how to fit a PIKO lighting kit to a G scale 2 axle baggage car which is a very similar process. If you're a fan of
schadenfreude watching the poor chap trying to wrestle the roof off it without breaking something during this live webcast was painful, it must have taken him nearly 5 minutes. Then he had an almost equal struggle trying to remove the plastic wheels by pulling them off with his fingers. (Hint, you need to lever them off with a rubber or plastic covered screwdriver or similar). Much cursing in German under breath I'm sure!