Cdi 270 head lights are dim

First thing: if the battery is 5+ years old, change it first. This is a German car and they aren’t forgiving about weak batteries. It will cause issues like this or strand you, so even if your issue persists you need to check this first. Mercedes uses a AGM battery in the trunk. Check the ground points around the headlights and the trunk battery as well, it’s not a common failure but it causes similar problems. While it is less common then the other failures these are somewhat known for the headlight assemblies getting "burnt" over time. If they look burnt buy some first and see — it’s a lot cheaper then the other things on this list.

Should all of that pass, there are two types of bulbs on these - Halogen or Bi-Xenons. If it's the Xeon trim, I would try replacing the bulbs first; it isn't cheap, but they do wear out with time - albeit at a much slower rate then halogen bulbs. If that does not work, the ballasts are going out. There are a lot for this car but they are interchangable, but don't expect it to be cheap - the 3 pointed star means a $50 part on a Japanese car is $150-200 because it came out of a Mercedes. Here's the list I found, but you can also get it from the ballast you have installed:

• A2038202526 – Central xenon module/control unit
• 1307329082 – Bosch/AL type ballast (also listed as 2285510600)
• 1307329088 – Another frequently used variant
• 10EEG050098, 10EEG091227, 10EEG091231, 10EEG112357
• 1307329066, 1307329068, 1307329072, 1307329073, 1307329074, 1307329081, 1307329084, 1307329086, 1307329087, 1307329089, 1307329090, 1307329091, 1307329092, 1307329095
• 2038208885, 2038209385
• A2208203785, A2208207085, A2208207185
• A2308208426

Take the headlights off and pull the ballasts out, and match it up against that list. Being Bi-Xeon, you might need a specific ballast that handles these. SM is not free but here you go: https://mbmanuals.com/series/203/ - The parts disparity cost is also why owners neglect them who didn't buy them aware of this, so often times the car has compounding issues you need to check.

If it's the halogen type, those age out badly with time and 95% of the time just changing the bulbs (both sides!!!) fixes it. If it's weak, it means there is a load on the car in the electronics dragging the battery down. Get a DMM with a 10A mode, put it where you can see the display, and pull the fuses until you see the draw go down. You might need to unlock the car and wait 30 minutes-1 hour before you do this (and don't bring the key) because Mercedes ECUs stay alive for 30 minutes-1 hour even without a key present. You need it at 50mA or less before you do the drain test. The MB SAM is very patient.