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...and want to find a radio that could have been in my 68SS from it's birth. So I'm probably looking for a junkyard specialising in US cars. There's one such junkyard in Sweden, but they don't have any.
Just change the one such junkyard in Sweden, to many junkyards in US, and then it would be true also.
If there were any 68SS in the boneyards, they were scooped up in 69. Your best bet is like Jack said, Year one or the like that sell repos. Either that or go to a big (super chevy) show, and hope they have one at the many flea markets that pop up in and around those events.
Might be easier if you change your attack pattern, and find out exactly what type of radio it was, and find out what other cars it came in. Might save you a buck or two, because your not looking for a 68SS radio. (like other people havent thought of this before, so it probably is useless advice)