From inside the trunk, unscrew the (4) studs that hold the corner of the fender on, I'm sure it has a name, then you will see the nuts that hold on the bezel.
If I remember correctly, I had to loosen my bumper and drop it down a bit.
It's officially called a quarter panel extension and 67mlbu is correct, there are four nuts that hold them on. There are body seals there and I don't know of anyone who makes them but most suppliers like Year One have sponge rubber gasket material for about $15. The tail lamp lenses themselves have gaskets as well and there are (or at least were) available for about $5 a pair.
Try Year One, OPG, The Paddock, and be sure to check shipping/handling charges. On a small order, those can kill you.
You definitly have to drop the bumber to remove the extenions which than allows you to remove the bezels.
Oh and don't ask what I paid for my NOS bezels a while back cause there not cheap.
67SS, If you want to get original equipment there is a guy down here in southern Mass with somewhere around 300 Chevelles in his yard. I've been down there a few times. It's almost like a Chevy museum.
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