GMC Sierra Owners Manual
For vehicles with heated mirrors:
(Rear Window Defogger): Press to heat the mirrors. If the vehicle has a towing mirror, only the upper glass of the mirror is heated.
The lower convex part of the towing mirror is not heated.
Depending on the vehicle's features, see “Rear Window Defogger” under Climate Control Systems (with Air Conditioning) on page 8‑1 or Climate Control Systems (with Heater Only) on page 8‑4 or Dual Automatic Climate Control System on page 8‑5 for more information.