I recently did a '89 GTE motor/harness/ecu into a '90 NA chassis. You basically need everything that attaches to your '87 harness under the hood and you might have to do some simple wiring on the ECU side of the engine harness to match up with the '90 chassis harness. The '89 is kinda the bastard child of the MK3s, so I discovered after I already had the motor and harness all in that the chassis side was a "little" different. One of the engine harness plugs didn't match up with the chassis plugs and there was one engine harness plug totally missing, leaving one chassis plug unplugged. Nothing on the unused plug had anything to do with the motor running, though, so no harm no foul. I tried two different '89 instrument clusters and neither would work with the '90 harness. All the plugs plugged in the same, but neither cluster worked. I wound up pulling the tach out of one and installing it in my '90 NA cluster so I could have a full functioning cluster, minus a stock boost gauge (a voltmeter is in the same spot on the NA cluster as a turbo cluster boost gauge). All in all the extra research and wiring I had to do was fairly painless. I expect you'll have the same engine to chassis harness issues as I did. Good luck!