Deet? That only works to keep the bugs off. Just kidding. 

Several years ago I tried about every remedy that I saw online. Toothpaste, WD-40, PlastX, buffing compounds, sanding discs, but only the sanding discs followed by the use of a buffing compound then application of a clear coat had any real effect and that was temporary… maybe lasting 6 months. I finally decided to replace both headlight assemblies with aftermarket ones from carparts.com and for $128 for the pair I finally got like-new crystal clear lenses and they still look new after 2+ years. I probably spent half that $ amount on all the other products combined plus the time spent cleaning, buffing and polishing. Lesson learned. Anyhow that was my experience so I can only recommend replacement if they are yellowed or frosty looking. If they are only slightly losing their clear appearance then either PlastX or a buffing compound may be the solution. But deet…? You would just be applying an oily coating that similar to WD-40 may provide a temporary less opaque appearance until it wore off. 

Just replace them, it's too burdensome to try repairing it, because it just keeps coming back. A cheap Chinese headlight is just fine. Is long as you have the proper insert, it's fine. 

nope

I've seen youtube videos of it.  

The concern I would have is that not all insect repellents have the same amount of DEET.  DEET is a plasticizer, and enough of it can melt/degrade nylon and several other plastics.  A really strong concentration might end up damaging the headlights more than cleaning them.