Could be that the ELD (in the underhood fuse panel) is toast. I agree that you should carefully check the grounds-but more important may be the ECU plugs. It really takes very little to cause damage or have some part that was corroded after 15 years cause the circuit to have more resistance than it should. I do have the Helm, but the drawings for an '89 may be off a little compared to your 90.
E-mail me if you need more help.