No power to both right and left low beams mercedes c240 2004

How did you test the bulb? A lot of modern vehicles use 'twin element' bulbs so you can still get a 'good' reading on a blown bulb if you only testing one element.. If you still have high beam but no low beam it is most likely the bulb or a short somewhere in the wiring.

Did you check the fuse circuit for voltage, use a testlite or something.
I posted part of headlamp wiring. You understand your dimmer switch is involved in the circuits.

Check for power with a test light with the light connected. If you have power then its the ground. If no power then you will need to trace the wire back to find break. As I recall they did have seperate fuse for left and right headlight high & low beam. Also did you get a defective new bulb?

Check the socket or connections that supply power to the bulb Sometimes the contacts can become corroded. Also, a loose or corroded ground wire can cause a bulb to not work or glow dimly.

both LOW BEAMS (you make this so hard not saying that...)
but challenging.....

is car in canada, or usa? or a?
does car have DRL. we dont know ,until you say.
and both types work different, non- and DRL.
my answer (roll dice, ) USA. no DRL

the system is simple (alldata.com)
2 fues, left and right
2 relays, one left, one right.
1 controller box.
1 switch combo.

the sequence of events
HL switch on,
this ground the Low pin on the mutiplex control box in fuse box.
this box sends 2 ground signals to both relays.
so if one headlight works you know the mpx box works
both relays close
and 12v is sent to both low beams , and there other side is grounded full time so they glow.
there are 2 fuse left and right. fuse #15 and 17

When either the high or low beam light is out in either side, DRL (daytime running lights) indicator will come on. When one bulb goes out, that's a clue the opposite bulb will burn out soon, so its best to replace both bulbs. If it was a fuse issue, both left and right side would be out. If driver side is OK but passenger side is not, then its a bulb issue. A 2006 Honda civic takes 9006 (HB4) low beam bulbs and 9005 (HB3) high beam bulbs. Cheapest is online from Walmart site-2-store. After procuring, wear latex, vinyl, or ANY gloves to keep your (corrosive) oil skin from coming in contact with headlight glass bulbs. Your skin oil will chemically react and cause shorter life span on glass bulbs. Instruction don't tell you this, so you handle with your hands, then need to replace bulbs 3 years later instead of 5 years (or more) in the future. With gloves on, pop the hood and look behind each headlight. Turn counter clockwise to unlock headlight bulb holders. Its a quarter to half turn to unlock bulb holder lock. Replace one bulb at a time so you don't confuse high beam from low beam. The wire has a tab you need to lift to pull out & disconnect wire connector. You might need a slim narrow-**** (NOT phillips) screwdriver to lift the tab; or use your house key. Snap the wire connector to the new bulb, then twist into headlight housing to secure bulb holders, then move on to the next bulb to repeat the procedure. Pict below, 9005 high beam is on the left, 9006 low beam is on the right. Handle by grasping the plastic base, not the glass bulbs. The raised potrusion on the bottom center is where the tab locks onto wire connector. Replace all 4 bulbs now, but in the future, its likely the low beam will wear out before the high beam. In the future, replace only the pair, not all 4. The factory installs inferior bulbs that go out sooner, hoping you'll take to dealership and they ******** you with higher parts-n-labor fees. As opposed to Walmart online site-2-store, it would cost you AT LEAST triple in parts & labor from dealership to replace all 4 headlight bulbs :-(

the bulb and fuses may be good, but are the bulbs in the right place, single in single socket and double contact in double socket ?

There is a fuse for left and right headlamps on most Nissans check them all. If you don't find that to be the case you can run a new wire from right low beam to left low beam.

For Audi A4 the fuse box is located on the left side of dashboard behind a plastic cover (for left hand steering wheel vehicle). On inner face of the plastic cover you will find the fuse diagram.
First fuse on first row from left upper corner is Fuse Nr.20 = F20 (15A) - RH headlamp low beam, headlamp adjustment.
First fuse on second row from left upper corner is Fuse Nr.21 = F21 (15A) - LH headlamp low beam, headlamp adjustment.

There are relays that control the headlights in a fuse box under the hood near the firewall.. There's also a fuse box underneath the dash on the driver's side, but can't remember if it's on the underside of the dash or beside it