Urr, actually, the spec changed end of 88 going into 89.
All the cars got the new interior (light grey) of the 90 spec cars
A year later the front bumper and rear spoiler were changed to 90 spec (i.e. drop down rear spoiler)
So, in reality, theres 3 different, original 86
changes put in 89 model year (actually available end of 88)
And the full on monty 90 spec
Hope that helps