My brake lights dim when headlights are onin 2004 yukon xl. When brakes are pressed blinker flashes fast and they take the place of the brake lights when headlights are on. If brake isn't pressed blinkers work fine. Brake lights are dim when headlights are off. Checked all bulbs and fuses and in Feb. This year had rear circuit board and turn signal switch replaced for this problem but still had this happen when it rained outside. I was told may be a bad ground but I don't know where to check this at.