One problem related to high/low beam switch has been reported for the 2004 GMC Sierra. The most recently reported issues are listed below.
The dealership was unable to install a relay switch because of a wiring change from the 2000 models. The low and high beam came on separately. When it snowed and the high beam was on, snow build up on the low beam. When this occurred it created a layer of snow, and consumer was unable to change back to low beam. The mechanics reported that there was and incompatibility's of the previously used relay. GMC would not correct the problem.