The whole movie was full of anachronisms. My friend, who got her masters in business/marketing, noticed that the Stoli bottles had the new labels, which replaced the older kind a few years ago - early 2000s, I think. In 1989 there would have been different labels on the Stoli. Also, there is Range Rover problem, which has been mentioned above. And the other cars, too. It's also been mentioned that Thelma and Louise came out in 1991.
Seems like they chose the year after they shot the rest of the movie, or Chris Columbus is too lazy to do period research. The latter is more likely.
Hear the words I sing: war's a horrid thing. So, I sing sing sing... Ding a ling a ling.