"The Durango-95 purred away real horrorshow - a nice, warm, vibraty feeling all through your guttiwuts. Soon, it was trees and dark, my brothers, with real country dark.
We fillied around for a while with other travellers of the night, playing hogs of the road. Then we headed west, what we were after now was the old surprise visit. That was a real kick and good for laughs and lashings of the old ultra-violence."
I'd say the 1977 Trans Am from Smokey and the Bandit (1977) qualifies in the same way as the 1968 Mustang Fastback from Bullitt (1968); they weren't "characters" and didn't "act", but they were tremendously popular. And technically I suppose the Torino from Starsky and Hutch qualifies as well since they made a movie based on the television series. And I know it isn't a car, but I'd also throw in the 1955 Peterbilt from the TV movie Duel (1971).
Edit: What about the 1959 Cadillac Ambulance from Ghostbusters (1984) and Ghostbusters II (1989)?