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Hi, new to this forum, but have owned several GM trucks over the years. I have at 94 K1500 with an odd rear light issue...

The front lights (headlights, parking lights, blinkers) and dash lights all work as expected.

When my headlight switch is off...the rear lights work as expected:
  • Brake lights work
  • Blinkers work
However, when the headlight switch is on, nothing works:
  • No tail lights
  • No brake lights
  • No reverse lights
  • No blinkers
Just to note, there is a 3rd light above the back window...I have never seen it come on, so I haven't done anything with it.
Here's what I've do so far:
  • I checked all the fuses on the panel right under the steering wheel. None are blown.
  • I checked all 6 taillight bulbs and they appear good (both filaments).
  • I swapped the headlight/parking light switch assembly with another, same result.
  • The ground wire on the frame near the back bumper was all rusty. I ground the frame and reattached the wire with new hardware.
  • I pulled the plug off the back of the left taillight and hooked a test light to it.
    • Hooked the clip to the black wire.
    • When the headlight switch is OFF:
      • The brake light wire goes positive when hitting the brake.
      • The blinker wire flashes positive as expected.
      • The taillight wire is off.
    • When the headlight switch is ON:
      • All of the wires are positive all the time.
Any ideas? I'm not sure what would cause this. I appreciate any info anyone has to share!
 

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Sounds like you lost the ground connection to the rear light assemblies. Easy check, pull all the parking light bulbs, both light assemblies. Then try the blinkers or brake lights. If the do not light there is a ground problem. The tail, brake and reverse bulb holders fail on that model. Could be an open ground in the light assembly.
 
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