Ebolanation

Not sure if this is where this goes but I have a 2006 ml350. My rear tail lamp has one bulb (left) when headlights are on that will not light. I checked fuses and cannot seem to find one that is burnt. If anyone knows which fuse number it is that would help because the paper I have for fuse location does not have it listed. I checked the ground an its good. I replaced the whole assembly and nothing. Please help me troubleshoot this issue. Mercedes wants to charge me 1200 to replace both rear assemblys. Why both? My service tech couldn't explain it other then they need it. But I told them I already did this and still could not give me a reason. Frustrated.