Overhead Brake lights not working on 1999 GMC Sierra 4x4?

First of all i will say,check the other lights.Are they working properly.
Does the high mounted stop lamp on the back top of the cab work? Do the both turn signals work?
If yes,then read further.

If all other lights working,but over head brake light not working with brake applied then first thing to be checked is Fuse.

If fuse checked ok,then brake light light bulb,the brake light switch and brake light wiring needs to be inspected.

But in most cases the fuse is the issue.
As per 1999 GMC Sierra Schematics there are 3 fuses that we first need to check. All 3 are located in the under hood fuse block. The first fuse labeled "Stop Lps" should have power on both sides of the fuse at all times. You can check for power with a test lamp or digital multimeter with the fuse installed. There are metal tangs that protrude at the top of the fuse, you should have power at both of these tangs. The other 2 fuses will require that you have someone apply the brake pedal to check for power. These two fuses are labeled "CHMSL" and "Veh Stop". Both of these fuses are powered with the brake lamp applied. Let's start there and see where we end up.

This will help.
Thanks.

See below latest solved car problems :----

Dragging brakes?

http://howtobyme.blogspot.in/2012/03/dragging-brakes.html

Brake pedal stuck?

http://howtobyme.blogspot.in/2012/03/brake-pedal-stuck.html

After replacing brake rotors and pads, brake pedal remains on floor?

http://technoanswers.blogspot.in/2012/02/after-replacing-brake-rotors-and-pads.html

How to replace Brake Power Booster?

http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-brake-power-booster.html

1997 Chevy Tahoe Brake lights not working?

http://technoanswers.blogspot.in/2012/02/1997-chevy-tahoe-brake-lights-not.html

How to service Drum brakes?

http://repairhelpcenter.blogspot.in/2012/02/how-to-service-drum-brakes.html

What type of problems usually noticed on Drum brakes?

http://repairhelpcenter.blogspot.in/2012/02/what-type-of-problems-usually-noticed.html

How to test Drum Brakes?

http://repairhelpcenter.blogspot.in/2012/02/how-to-test-drum-brakes.html

Brake Bleeder Screw Function?

http://repairhelpcenter.blogspot.in/2012/02/brake-bleeder-screw-function.html

Replacing ABS unit on Buick Century?

http://technoanswers.blogspot.in/2012/02/replacing-abs-unit-on-buick-century.html

How to Deactivate Airbag system on Ford Ranger?

http://technoanswers.blogspot.in/2012/02/how-to-deactivate-airbag-system-on-ford.html

Brake pedal is pulsating and vibrating?

http://repairhelpcenter.blogspot.com/2012/01/brake-pedal-is-pulsating-and-vibrating.html

All types of brake problems and repairs

http://howtobyme.blogspot.com/2011/11/abs-and-service-light-comes-on.html

What is Brake Drum Bar?

http://repairhelpcenter.blogspot.in/2012/02/what-is-brake-drum-bar.html 

ABS light stays on but brakes work fine?

http://howtobyme.blogspot.com/2011/11/abs-light-stays-on-but-brakes-work-fine.html

VCS maintenance required on Toyota Camry?

http://howtobyme.blogspot.com/2011/11/vcs-maintaince-required-on-toyota-camry.html

VCS and ABS lights go on and off intermittently on Lexus?

http://howtobyme.blogspot.com/2011/11/vcs-and-abs-lights-go-on-and-off.html

ABS & Brake lights warning indicator steady on dash and speedometer not working for Dodge Ram?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

Isuzu rodeo speedometer is jumping?

http://schematicsdiagram.blogspot.com/2012/01/isuzu-rodeo-speedometer-is-jumping.html

E-brake is stuck down on Ford Focus?

http://schematicsdiagram.blogspot.com/2011/12/e-brake-is-stuck-down-on-ford-focus.html

Brake warning light comes on and car will not start intermittently?

http://repairhelpcenter.blogspot.com/2011/12/brake-warning-light-comes-on-and-car.html

When the car stops, abs pump turns ON?

http://howtobyme.blogspot.com/2011/12/when-car-stopsabs-pump-turns-on.html

Where is Power Brake Booster located on 1995 Lincoln Town Car?

http://howtobyme.blogspot.com/2011/12/where-is-power-brake-booster-located-on.html

Sometimes ABS light comes on while Driving?

                     

http://howtobyme.blogspot.com/2011/12/sometimes-abs-light-comes-on-while.html

How Anti-Lock Brakes Work?

http://technoanswers.blogspot.com/2011/11/how-anti-lock-brakes-work.html

Brake System Troubleshooting?

http://technoanswers.blogspot.com/2011/11/brake-system-troubleshooting.html

Car Brake lines have no pressure?

http://schematicsdiagram.blogspot.com/2011/12/car-brake-lines-have-no-pressure.html

Car Brakes Dragging?

http://repairhelpcenter.blogspot.com/2011/12/car-brakes-dragging.html

Car Brakes grabbing?

http://repairhelpcenter.blogspot.com/2011/12/car-brakes-grabbing.html

Car Brakes Making Noise?

http://repairhelpcenter.blogspot.com/2011/12/car-brakes-making-noise.html

Car Brakes Pulling?

http://repairhelpcenter.blogspot.com/2011/12/car-brakes-pulling.html

Pulsating Brake Pedal?

http://repairhelpcenter.blogspot.com/2011/12/pulsating-brake-pedal.html

Spongy brake pedal?

http://repairhelpcenter.blogspot.com/2011/12/spongy-brake-pedal.html

Hard Brake Pedal?

http://repairhelpcenter.blogspot.com/2011/12/hard-brake-pedal.html

How to Disconnect Power to EBCM on Pontiac Grand Prix?

http://schematicsdiagram.blogspot.com/2011/12/how-to-disconnect-power-to-ebcm-on.html

Where is ABS/TCS module located on 2005 Cadillac Deville

http://schematicsdiagram.blogspot.com/2011/12/where-is-abstcs-module-located-on-2005.html

How to Replace Power Brake Booster on 1996 GMC Jimmy?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-power-brake-booster-on.html

Isuzu Trooper Shifts hard and Engine Revving in stop?

http://schematicsdiagram.blogspot.com/2011/12/isuzu-trooper-shifts-hard-and-engine.html

What is Brake Bleeder Screw?

http://repairhelpcenter.blogspot.com/2011/12/what-is-brake-bleeder-screw.html

How to replace car Brake Cable?

http://repairhelpcenter.blogspot.com/2011/12/how-to-replace-car-brake-cable.html

How to Replace Brake Disc?

http://repairhelpcenter.blogspot.com/2011/12/how-to-replace-brake-disc.html

ABS motor keeps running?

http://schematicsdiagram.blogspot.com/2012/01/abs-motor-keeps-running.html

Anti-lock brake light stays on all the time?

http://technoanswers.blogspot.com/2012/01/anti-lock-brake-light-stays-on-all-time.html

Is it necessary to replace Brake caliper while replacing rotors and brake pads?

http://repairhelpcenter.blogspot.com/2012/01/is-it-necessary-to-replace-brake.html

Emergency brake Adjustment for Hyundai Tiburon?

http://schematicsdiagram.blogspot.com/2012/01/emergency-brake-adjustment-for-hyundai.html

How does Brake Backing Plate works?

http://repairhelpcenter.blogspot.in/2012/02/how-does-brake-backing-plate-works.html

How to replace control panel on Audi A4 models?

http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-control-panel-on-audi-a4.html

How to inspect, service and replace brake shoe liner?

http://repairhelpcenter.blogspot.in/2012/02/how-to-inspectservice-and-replace-brake.html

How to inspect brake drums?

http://repairhelpcenter.blogspot.in/2012/02/how-to-inspect-brake-drums.html

How to replace Brake light switch on Chevrolet Tahoe?

http://technoanswers.blogspot.in/2012/02/how-to-replace-brake-light-switch-on.html

How to replace brake rotors?

http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-brake-rotors.html

Engine accelerates when brake is applied?

http://technoanswers.blogspot.in/2012/02/engine-accelerates-when-brake-is.html

Loss of acceleration?

http://technoanswers.blogspot.in/2012/02/loss-of-acceleration.html

1996 Chrysler LHS brake light circuit wiring diagram?

http://technoanswers.blogspot.in/2012/02/1996-chrysler-lhs-brake-light-circuit.html

How to replace rear brake pads on 2002 Lincoln continental?

http://technoanswers.blogspot.in/2012/02/how-to-replace-rear-brake-pads-on-2002.html

Brake pedal firm when engine not running goes to floor after engine starts?

http://technoanswers.blogspot.in/2012/02/brake-pedal-firm-when-engine-not.html

How to replace rear Brake lights on 2004 Toyota RAV4?

http://schematicsdiagram.blogspot.in/2012/02/how-to-replace-rear-brake-lights-on.html

Brake drag from wheel area while driving?

http://schematicsdiagram.blogspot.in/2012/02/brake-drag-from-wheel-area-while.html

 Brake pedal Pulsating?

pulsating.htmlhttp://schematicsdiagram.blogspot.in/2012/03/brake-pedal-pulsating.html

Brake pedal travels excessively?

http://schematicsdiagram.blogspot.in/2012/03/brake-pedal-travels-excessively.html

How To Repair Grabbing Brakes?

http://schematicsdiagram.blogspot.in/2012/03/how-to-repair-grabbing-brakes.html