Cosmetic upgrades for '89 that bolt straight on:
Side mirrors
Front bumper cover
Fog lights
5 spoke wheels from a '91-'92
Belt molding trim, if you clean up the mounting tabs from the metal trim pieces first, but it uses the same provisions.
Upgrades that require a little work
Front signal lamps and grilles (requires front nose panel support)
Tail lights and center finish panel (requires modification for lock cylinder)
More work needed:
The 3 piece spoiler requires that you drill some holes. Also, if you use the wing brake light, you should change out the tail lamp failure module with one that expects the LED stop light. Alternately you can connect an 1156 lamp to the stop light connector, but that's just ugly.
The '89 steering wheel requires that you also swap in the combination switch, if you want the cruise control buttons to work, I think that the other buttons can be hacked to work though.
Apparently the combination meter (dashboard) is wired slightly differently as well, requiring that you move a few pins around in order to get power to things.
Other minor differences exist, like the rear shoulder 3.5" speakers, but that's probably getting outside of the scope of the question. the '91-'92 6.5" front speakers just need the speaker mounts, but it would be a good idea to use the door panel as well.
Not hard at all to make the exterior of an '87-'88 look just like an '89-92. The USA Spec '91-'92 is harder interior, because you don't really want the airbag with nothing controlling it. Canada didn't get the airbags, so it's the same as '89 that way.
