The ECM from a 90-92 will work on an 89 IF THE ECU AND HARNESS ARE THE SAME COLORS.
87-88- ECUs all the same, difference is turbo and non-turbo
89- A Mix year. Some came with a 87-88 harness and some have the 89+ harness. You will know the difference by checking the color of the body harness. When you look in the area of the ECU's location, you will see a few body plugs, if they are yellow, you need a 87-88 computer. If the plugs are grey you can use the 89,91,and 92 computer.
If your harness is yellow, I have a computer for sale. If the harness is grey, I also have a computer for sale.