Corner lights short out when turned on. 67 c20
 

Ok I have a 67 c20 2wd lwb it is bone stock base model no option truck. When I turn on the park lights the fuse blows. I have replaced the head light switch with 2 different new ones and it still blows. I have brake lights until I turn on park lights. All other circuits are ok and the is no add on parts under the dash tied in. I do have a trailer brake controller that will activate the brake lights also. Any help would be great as it's my only rig and the days are getting shorter.
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Re: Corner lights short out when turned on. 67 c20
 

Most likely in the rear light system. Disconnect the round plug connector that has four wires (3 green and a brown, brown is for tail lights) on the firewall and see if the fuse doesn't blow when you turn on the lights.
If the fuse doesn't blow, then check along the rear light harness along the frame rail for bad (exposed wires). Could also be in your trailer connector.
You can also disconnect the plug back at the end of the frame rail at the rear, to eliminate that section.