My brake lights, turn signals and hazard lights aren't working.

SOURCE: No tail lights, brake, hazard and signals work.

You did not mention if you found the fuse for "tail lights"..
Allot of times there is two fuse blocks.. One under the hood and maybe also under the dash..
Also - check the maual for where the relays are..

A good resourse is autozone.com
Go there and resgister,, put car info in and there is a worth of info like - locating parts on car,, how to and diagrams..

Good luck ;o)

Good luck...

SOURCE: o6 GMC SIERRA FUSE BOX

Check trailer wiring, That's a good place to start.

SOURCE: bottom brake lights and hazard lights do not come on. All other lights work including top brake lights. Fuses and bulbs checked.

Check for power out of brake light switch by brake pedal. Of its faulty , the 3rd brake light will still work. Hope this helps

SOURCE: TURN SIGNAL, BRAKE & HAZARD LIGHTS

No Turn Signal Operation (blinker)The turn signal lights in your car are a basic electrical system that controls the front and rear blinker lights to alert the intended direction of the vehicle. We have listed some of the most common problems and repair troubleshooting tips below:
Troubleshooting Procedure
To start troubleshooting this problem:

• Turn the key to the on position (do not start engine)
  
• Inspect the system fuse located in the fuse panel and replace if needed. If you cannot find the location of the blinker fuse consult your owner's manual or an online repair manual
  
• When a blinker flasher fails it can cause blinker lights to say on, not blink, or not go on at all. To test the blinker system flasher, remove the flasher unit and insert a jumper wire between the two terminals. If blinker stays on in both directions without blinking, replace the flasher. If you cannot find the location of the blinker flasher consult your owners manual or an online repair manual.
• 
  

SOURCE: 2005 sierra horns stopped working.

The click you hear is the horn relay -- this likely has either dirty / corroded connections or burned contacts inside the relay. If cleaning all relevant connections doesn't solve the problem, replace the relay. You can find the realy by having someone push the horn button while you track down the click.

Charlie