Ronnie was, in fact, correct in that the panel light adjustment was the issue. The '90 Reatta has a lever type adjustment for the panel lights. The lever is a flimsy piece of plastic which requires that it be pushed inward and then slide to the desired position. You need to be light figured to avoid breaking this lever. Evidently the lever was not seated properly which prevented the instrument lights to come on when the headlights were activated. I also noted that the lever needed to be positioned to the right of the scale mid-point in order for proper operation. Case closed.