I've seen some strange ones on I-5 too. A few years ago I was heading back down to L.A. from Lemoore, and right at the bottom of the Grapevine, I saw a 1973-ish Ford LTD Country Squire heading the other way. I honestly laughed because it was just something I really didn't expect to see, especially out there.

Even stranger yet - I saw a 1976 Honda Accord hatchback way out in the middle of the nowhere on I-40 westbound a couple of years ago, around the Arizona-New Mexico border. It was seriously beat up, but hey it was being driven and apparently able to handle long-distance drives.