12.629.718 is Fiat's part number. Looks like there's some demand for the tail lights. 3 of the left side without the backing or the reverse lens went for $41 about a month ago. They're NOS, which yours are close enough to being if you throw the newspaper in with them.
70 71 72 73 Fiat 850 Spyder Spider tail light lens NOS* - eBay (item 190467065473 end time Nov-17-10 17:49:13 PST)