One problem related to brake light on has been reported for the 1999 GMC Sonoma. The most recently reported issues are listed below.
During a panic stop the brake system light came on and the brakes felt soft but still worked. During the drive the brakes became very weak. When I checked the brake fluid it was empty. I refilled it and started checking for leaks. I found that both the front and rear brake lines we fractured because of the pressure during the panic stop causing the rusted lines to burst. This seems to be a known ( but not reported) problem for this truck in the northern area. .