There is one thing in my house that I always forget to clean, and by the time I notice it, it has gotten pretty nasty. Thankfully, I'm not talking about anything that you would think to look at much of the time, and perhaps the reason it gets missed is because it is not something anyone pays any attention to. I often notice it when I am lying in bed, and that is because that is when I can see it. I notice that the light cover over the overhead light is looking a little dull and there may even be a few dead insects in there.
If you don't have an open light cover, you probably don't have to worry about the dead insects, and it will probably only need to be cleaned on the outside. Many of them are open however, and as we all know, bugs are attracted to light. Sometimes they die up there, and you can see vague brown or black shadows through the light cover. Top this off with the dust that can accumulate and you have something that will drive you nuts until you clean it.
The problem with cleaning a light cover is getting to it. If you have high ceilings, you may even have to get a stepladder to get to it. I have broken more than one light cover trying to take it down and clean it, and I don't look forward to doing it. In fact, they all look like they could use a good cleaning right now. Make sure you have a steady perch when you have to climb up to take it down for cleaning, or you may end up dropping it. Every time I have dropped one it was because I lost balance and had to either catch myself or save the cover. I don't really want a broken leg, so the cover was dropped every time.
For the most part, a simple glass cleaner will clean up a light cover just fine, and you may find that is all you need for the ones on your car as well. Don't forget that you need to clean them on occasion so you can see at night. If you find that the dust seems to have formed into a thick layer that won't easily come off, you may have to soak them to get them clean. Fill your sink with water and dish soap and soak for about an hour, and your light cover should come out nice and clean.