This is interesting. According to the 2001 MB S-class brochure, this vehicle comes with xenon fromt reading lights to " provide a quality of light that more closely resembles natural daylight. I immediately made the jump to WOW miniature xenon discharge klamps just like the headlights. Wow what an expensive solution to a reading lamp. Wrong!. They came on and looked very mucht like regular incandescent bulbs. Well...being an old time chemist with access to all kinds of bizarre equipment I used a portable spectrograph and inspected the light with that. ( I probable could have just as well checked the bulb for a filament but this was more fun)! Indeed the light showed the typical continuous spectrum just like any other incandescent lamp. I guess the xenon is the filling in the envelope.:)
As I had the instrument, I checked the headlights and saw the typical lines of a gas discharge lamps with lines in the indigo, blue green etc to make the strong blue-white
glow.
That's the story! :)

There are accessory bulbs that contain Xenon. I think Philips makes one but forgot the bulb type. There are some aftermarket companies such, as Raybrig/Stanley that put Xenon gas in miniature bulbs also. They cost about 3x the price but, it is kind of old news. Osram/Sylvania also makes some that supposedly contain either Krypton and/or Xenon and I use those for brake lights. If you want to learn more or find out if you want Xenon reading bulbs. Go to the Philips Automotive Lighting Forum. All they talk about is automotive lighting.