Nick is correct, 1990 was the year of the change. Another difference is the seats. I believe in 1990+ the leather seats became standard. They have 3-4 broad horizontal strips on the upright portion of the seats (against your back). The 1989 and earlier models tend to have a tough,leather-like vinyl surface with narrow vertical strips. (Leather may have been an option, though.)
I have also heard some of the later '90s have upgraded 6-speaker sound systems. Mine doesn't.
FYI, I was looking at '89s 'cause they were cheaper until I sat in the '90 seats. Much more comfortable, IMHO.