91 Comanche 6 cyl/ 5 spd/long bed 4x4. Having issues w tail lights. As you know, from top to bottom, 3 segments. On both sides: Top : brake light and turn signal. Middle: also brake light and turn signal, Bottom: back up light. All are working,but w headlights on, I have no constant tail light. ( Brake and blinkers continue to work w headlights on). I can't figure this out. changed all the bulbs, refreshed ground wire behind driver's tail light. Could this be the headlight switch? Which segment should light up w tail light function? Thanks in advance. Ken