Well, pretty much everything. I mean, engine-wise the differences are moslty obvious. '85 -> 2.2, log intake, PM/LM computer. '89 -> 2.5 common block, 1-piece intake, SMEC. '85 supposedly had a poor distributor setup. The '89 will have the better transmission. Body-wise, the fenders should work but the ground effects are different. Wheels will swap. All the interior parts will swap. The rear hatch technically is interchangeable, but the spoiler is different. T-tops are the same on both cars. I haven't dealt with 1st gens a lot, so I don't know if the brake system changed. I'm sure it's easy enough to figure out though. They should both have 10" front rotors, but the '85 will likely have rear drums whereas your ES is likely to have rear disc.