The reversing lights on my Skyline were driving me NUTS! over the years it had spent in Japan the light lenses had turned yellowy GROSS!
This is a common thing with import cars & some old cars. something to do with whats in the air & what the plastic absorbs when it expands & contracts etc.
Ok the things you will need to do this
800G wet & dry sand paper
1200G wet & dry sand paper
Metal Polish – like Brasso
Mentholated spirits or rubbing alcohol
Take any electrics out of the lights.
Grab a bucket with some water in it with your 2 types of sand paper.
Start with the 800G make sure you wet the paper & the light before sanding.
You will see all of the yellow gunk has pretty much already come off.
Once you get similar results as this switch to the 1200G
Once you’re happy with how the light is looking rince it off
Now comes the polishing part
Go crazy with polishing so you get a nice clear finish
Now clean the light with the Mentholated spirits or rubbing alcohol
If you got any water inside the light pour some inside the light & give it a good shake & then poor it out. the Mentholated spirits or rubbing alcohol will evaporate any moisture in there.
& you’re done!
Here is a few comparisons
Before & After