Ok, I have a 93 500SEL which I converted to eom xenon lighting after acquiring a set of lamps from a smashed 97 S500. First you forgo the self leveling feature unless you acquire both axial sensors and the control box.

Second I wonder about the quality of the light output from those china lamps. Actually installing a set of OEM lamps from a 95 will give you better quality, near xenon in fact if you run special H9 burners on H7 bases. You don't mention if you have Day time running lamps, this makes a difference. Lastly Mercedes altered the wiring for headlamps many times in the 92-96 period so your car might be different.

For the pre 95 lamps they would be H4 (high/low)/ H3(fog)(I believe, maybe h1) for the 95 they would be H7 (low) H1 (high) H1 (fog) with a city bulb in Europe to provide a few watts of lighting in a well lite city at nightime. This city lamp socket likely is the extra bulb position you are talking about.

For the Xexon the oem reflector is a different part number, the glass lens is different, and it provides D2R, h1(high) h1(fog). I've a spare busted xenon assembly, plus two 95 assemblies in the garage, so I've checked this first hand.

Now the issue with the pre 95 is the xenon run on the low beam power, but when you switch to high beams, you switch power from low to high and lose power to the xenons, this is not good. Post 95 you would find the H7 is on all the time which is more workable. Even for those projector lamps using non-xenon you have the same issue however the lighting you get from running high beams only (versus high and low) might be acceptable and switching the (non-xenon) low beams on/off isn't an issue

You could rig power and relays and control from the parking lamps to feed the xenons but then I'm not sure if disconnecting both low beams wouldn't turn the lamp out indicator on, might not, you could test that first, by disconnecting both low beams and see what happens. Mind finding 30 amps of power in the engine bay is a chore.

So what I found was the feed from the lamp switch goes to a connector under the steering wheel which feeds the wiper control stalk. The stalk logic provides power either to the H4 low (yellow), or to the H4 high (white) depending on position of the stalk, this feeds to the fuse box where you have 7 amp fuses for each circuit. If you pull it back to flash you feed power to both the high and the low side where the high side comes from I believe the feed from the rear window heating element.

For 95 onwards I think the lamp switch feeds the fuse box directly for the low beam, and feeds the control stalk connector which only provides power to high beam when needed.

Oddly in my case the wire was marked as being 16 gauge from the lamp switch to the connector, but was actual 12 gauge, so we soldered in a junction block to provide power to the yellow feed going to the fuse box, and then still fed the stalk connector which provides high beam power when needed. Plus then swapped the 7 amp fuses out for 15 amp ones.

I'll note if you have (daytime running lamps) DRL you need a relay to prevent feedback from the yellow DRL to keep it from providing power to the high beams, otherwise the DRL will attempt to back feed power to both the high and low beams based on stalk position, which is not a normal condition and will overload the DRL relay.