I have cracks in my tail light lenses and would like to know what is a good filler for light surface cracks? I'm restoring them for my 71 Duster 340. Its cracked in an area that gets painted black. So what works good? Should I polish them first then fill or vice versa?

Since it's a painted area, you might look at Plastifix.

Regarding polishing, I would think, repair the cracks, polish, then paint where required.

I agree on the plastifix in the painted areas.

If repairing a clear or colored lens in the visible areas is pretty much impossible. There are glues and products out there to glue them together etc. but most are for patching stuff together short term, not meant for "restoration" type repairs, most if not all of them will fail over time due to the hard/heat/chemical resistant properties of the thermoplastic that tail lights are made from not allowing a good bond in the first place. Even if they do work you'll always see the repairs. I'd suggest finding a better used lens to start with if you want to skip the $350 NOS examples on ebay/ 70/71 Duster lenses in good used condition are pretty common and not usually real expensive, they tend to have a pretty high degree of survivability.

I'll try that. The cracks are only where the blackout sections are, the illuminated parts are crack free. They should turn out good.