It's easiest if the wiring harness, ECU and chassis harness are all the same. So if you've got an '89 car, then it's best if the wiring harness and ECU are also from an '89. The ECU and engine wiring harness must match, but they can be different from the chassis harness with relatively minor issues.
However, I have the wiring harness and ECU from an '87 in my '89 car, and there were very few issues. The ABS connector was different entirely, and the headlight relay needed to be re-wired.