Hello Swede,
I'm pretty good at internet searches but I have come up with nothing on this. I need a few more clues, particularly the artist's first name. Whoever he or she is or was, they apparently were not too prominent.
I have a suggestion. You may have already done this. The major newspaper in Stockholm may have its archives available for searching. (I have access to the New York Times back to 1865 or so.) Searching or viewing the daily papers from around the date the event took place may provide more information. Or if they have a good search engine try searching under "Fiat" "Preston" and "Skansen" and narrow the date down to 1977.
Are Fiats still sold in Sweden? If so they may have an office in Stockholm that might help. Couldn't hurt to ask. Oh, I see they have a website, you have probably been there already:
http://www.fiat.se/cgi-bin/pbrand.dll/F ... VCookie=no
I can't read it of course, but I'd look for the address and phone number and see if I could talk to someone. If you can find a contact within the company you could email the pictures you have and see if they come up with anything.
Good luck. Let me know what you come up with.
ps. If you still come up with nothing, let me know. I have one other idea you can try.