I have been researching this online - there are whole research papers on the subject and seems that solvents are one major cause on clear polymers such as acrylic (PMMA), and polycarbonate, so don't clean your lights with alcohol/polish with petroleum distillates etc...
So this only happens to lights with a clear outer layer of specific materials such as that favoured by Audi, it doesnt affect the brittle red material usually used on most cars' rear lights.
It is a shame.
Anyway, I left my light lenses in the oven overnight on platewarmer setting and they have cured nicely so will be gluing them back together this morning and fit them this afternoon if the rain stops. If not, will just leave them off until another day and tape over the cable hole, the outer lights seem sufficient/legal.
Another interesting point is that the direction of the cracks always comes from the small circular dot. It is in the middle of the reverse lights on ours and look on the next phot, comes from a similar dot on the lower right.
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