Mine are not yellowed but are clouded on top with thousands of tiny stress cracks. 3m and I are unable to restore back to clear. There not terrible but I feel need replacing as this is the only issue on exterior of the car. I see on E bay the 06 xk's and earlier hid's can get a lenses with no insides but for the adaptive on the 07's and on I don't see this option on the net. Any idea's on where to locate new lenses without buying entire unit complete or does this option not exist yet? Thank you in advance for your time.

 

had scratches and yellowing on my 2007 XK. Had a body shop wet sand them for $100 and they look great now.

Ron

 

...and clearcoat with UV inhibitor clear so it lasts.

 

Had exactly the same issues with mine. Wet sanding and UV clear coat is the fix.
A hundred bucks is about the going rate here for a quality job. You may be able to find a pro detailer to do it for less but make sure he/she is well regarded in the field.

 

Yep I tried the wet sanding with little effect as the stress cracks are more than surface deep.