Forgive me it this has been addressed, I tried the search box and nothing came back.
My left outside mirror gets very little heat when the defrost is activated compared to the right mirror.
Vehicle is a 2021 King Ranch Powerboost with power folding mirrors, moonroof...has most available accessories and options except the max tow and it has the smaller 120 volt bed mounted outlet.
What's a good way to diag that condition?

 

What I've done in the past to diagnose this on other vehicles is remove the glass from the mirror, there's typically 2 wires connected to the glass for the heat option. I disconnect both wire and hook up an incandescent test light to provide a load, have the vehicle running and make sure when I turn on the heated mirrors the test light is lit brightly sometimes I also tap in a multimeter to check the actual amount of voltage going to those two wires while it's under load. If the voltage is over 12 volts. If the voltage is at the required spec then there's something wrong with the glass if it's not then you know it's a wiring issue in the vehicle and if I'm not sure I checked the voltage on the side that's working. I just had a 2022 Sierra with the big tow mirrors that was full of corrosion on the plug that plugs into the mirror.