Yep that's the breather cap. I actually just discovered that it's been causing moisture problems in my passenger headlight. It's been getting moisture in it after it rains for a while but dries out shortly after. Over the winter I went around the entire lens with clear silicone in an attempt to seal it but water was still getting in. There has also been a small piece of plastic in the light for a while but I never figured out where it came from until I pulled the breather cap off today. The plastic piece chipped off the hole for the breather causing the hole to be bigger than the cap and allowing moisture to get in the light. I'm gonna go around the hole with some silicone and hopefully that fixes the leak. Maybe that might fix your guy's problems too!