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The brightness is the same all the time. The power to the headlights is not reduced by a resistor the low beams are full on and operating at the full battery voltage. There might be a resistor melted to short circuit somewhere. I still have not been able to locate or test the DRL resistor if this vehicle even has one. I have looked all across the rad support and around behind the headlights but no luck there. I saw a video of a tech locating a DRL module on a E250 so I think I know what it might look like. Covers not installed for this picture
I wonder it that auxiliary relay box for the DRL relays is the "spliced-in" version instead of the factory "plug and play" version. If it's the spliced in version, that could be a key bit of data that might allow you to figure out what's going on.
Is there anything buried out of view behind the air filter? It's obscuring the view of the location where the DRL resistor, if present, is supposed to be. If the relay box was spliced in, then the installer may have omitted the resistor and wired it up so the DRLs would run at full instead of reduced brightness.
I would not worry about the "sun sensor", it's part of the autolamp system which, as I already noted, should not be capable of causing your symptoms.
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