Hey Ive got 3 set of head lights with the cracks in them. 1 set of ebay projectors for a chevy s10, (bought used) and then I retrofited them with valeo projectors. a set of standered stock GMC jimmy lights and a set of GMC envoy lights HID. from what ive read this has nothing to do with heat im like 85% to 90%sure of that, It has everything to do with the plastic being exposed to UV light, and the way the plastic breaks down and oxidizes. I sanded them out of the envoy lights and what I could not sand out became transparent after I sprayed automotive clear coat on the outside of the lights. When the plastic is exposed to UV it starts to break down then becomes brittle and cracks, from what I understand it should not be caused by heat unless it is vary cold out and the lights heat up and cool down repetedly. For my envoy lights this was not the case as I bought them from some one in L.A. Ca. and the truck was there from new till it was totaled. I have also heard that if the plastic lens's are kept from contact with the air this should not happen. I dont know about plastic film being applied to the lights but I know that the ebay projectors I have were sprayed with clear coat when they were made both inside and out, I can see the runs in the clear on the inside of the head light. Gm also did this and the jimmy lights cracked where the clear coat had been lost due to oxidation. I would think that if you were to take the lights apart and sand them 2000 grit paper. then have them spraye