Being aware of this issue back in 1988 (!) when I found 2 crashed Fulvia in the local scrapyard I removed them by cutting them out. This allowed the nuts to be cut off carefully and the rusted threads cleaned. I suspect that they'll be rusted again now! But as Chris says, once the bolt head inside the lamp unit starts turning it's the end of it. Disassembly is possible but bending the tabs back is fraught with risks! Easier to replace with new. I'll have a look see if I've got one Chris! If you wanted a new pair Gerry has a small stock at the moment of newly made ones.
Guy