@mngrif Never tried it but I can tell you why it probably isnt bullshit. DEET is a strong solvent and will dissolve plastic with ease. IT is also a plasticizer and will permanently soften plastic. This means two things

1) if your headlight cover is plastic like most modern ones are it will take off a layer of the plastic. This may produce a shine (though it may just ruin it and make it fofggy looking, not sure)

2) if your headlights are made of glass it will dissolve everything off the surface and leave the glass. This might give it a nice shine.