Just curious if anyone knows how easy this would be? I have the 87 interior harness as well.
I imagine its as simple as cutting and splicing the 87 interior harness into the 84 harness.. It's only 10-15 wires for the cluster and stuff that run up to the dash..Both transmissions are 5 speeds

I put an '87 2.5L into an '86 3 months ago (both 5 speed). I plugged the existing cables in the console from the dash into the '87 ECM & everything was fine. It's good to have the '87 ECM mounting bracket because the harnesses plug into the side instead of the bottom. You may already know that you have to do some fuel line rerouting and change the water pump outlet for the heater hose...

You are not going to just be able to plug the 87 harness into an 84 body. The 84 is wired differently than all the other Fieros.

C500 (the plug near the battery on the 87) is near the center front firewall in the 84. The 87 uses a power distribution post under C500. The 84 uses the starter positive battery terminal as a power distribution post.

I would recommend modifying the 87 engine harness to plug into the 84 body harness with the exception of adding the power distribution post to the 84 body. GM realized that using the starter terminal as a power distribution post wasn't a good idea.

If you are ok/good with wiring you can do it yourself. If you want I would modify the 87 harness at a charge.

[This message has been edited by phonedawgz (edited 02-03-2014).]

Edit: oops, you're putting in an '87 Duke. I misread that as an '87 V6 for some reason.

[This message has been edited by Blacktree (edited 02-03-2014).]