I have a 2018 Chevrolet Silverado 1500 crew cab LTZ Z 71. All stock.
Had same left turn signal failure alarm and would blink fast after a few seconds. All lights in rear working.
1. Changed bulb-same
2. changed harness-same, did notice one of six pins missing in male plug when plugging in new harness. Didn’t follow this to front. Not sure how far it goes before next junction.
thanks!