I think your right. I don't recall the lighting on the rear of your truck, but if you have a different bulb for sig and brake (on each side), it is probably the flasher unit. Some trucks (like I said, don"t recall the Silverado), have separated sig and brake circuits (quite sure the Serria is separate, I remember the sigs being amber). One of the jobs of the flasher is to output these two leads into one for the trailer connector. Try a new flasher, that will probably solve the problem.