The Pittsburgh Post Gazette is cataloged on microfilm in it's entirety at the Carnegie Library of Pittsburgh. Unfortunately it's only sortable by date, so it's pretty much ask the librarian for a week's worth of papers on film, and then go sit at a viewer trying to find that certain ad. The "mid-80's" is a lot of papers to look through. It's not impossible, but it'd take a few days of sitting in the library for sure. If you had a real good idea when the ad ran it would be a lot easier to track down. Probably wouldn't be of much help once you found it anyhow, car is probably long gone and the phone number changed. Sure would be cool if you dialed it up and tracked down the owner though.
One should only be so lucky.