It could be the flashers that are being installed. Normal flashers cannot handle that big of load.
Your profile does not show year of your Sierra. It would be good to know that to research whether yours have an available heavy duty flasher.
Do you have trailer package? You could look under the hood for those fuses. The trucks wiring works relays under the hood fuses and relay panel. These only work if the truck has towing package and the trailer is connected to those wires rather then being tapped off truck's lighting wiring.
You will get a lot of input about switching to LED lamps.
