76 k15 strange lighting issues

[76 gmc k15]

Hello everybody, on my 1976 k15, when I push the brake the gauge backlights come on and the front and front side markers come on. Also when the right turn signal is on all the turn signals flash and the right front side markers does not flash. Also when the 4 ways are on the front side markers do not flash. I also found these wires in between the cab and bed, the white and green one run into the cab and the yellow is just attached to a bolt on the bed. Also there is a wire under the dash that when you push down the brake and then touch it to metal it will cause a short but it is only hot when the brake is pushed

 

[76 gmc k15]

I forgot to add
Thank you any help is appreciated

 

[Snoots]

ST_352_76_1976_GM_Light_Truck_Wiring_CK_10_30_Only

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[SirRobyn0]

@76 gmc k15 Is this truck new to you?

I think you need to look at those wiring diagrams, but just in the pictures you have share obviously there is wiring for something disconnected and that flasher unit looks like non-OEM wiring and connectors so if this truck is new to you who knows what the PO did. I will say in general, often it is grounds causing lighting abnormalities, but if someone miswired something that could do it to. You need to look at diagrams, trace wiring and look at grounds. Without being there with you it's going to be difficult for any of us to guide you along, but maybe some will see something else in your pictures, and get an idea.

 

[76 gmc k15]

Thank you, sirrobyn0 and snoots, a friend mentioned to me today that the wires between the cab and bed could have been for a accessory such as a fuel tank with electric pump

 

[SirRobyn0]

Thank you, sirrobyn0 and snoots, a friend mentioned to me today that the wires between the cab and bed could have been for a accessory such as a fuel tank with electric pump

Possibly, could have been for lighting or other electrical in a canopy too.

 

[75gmck25]

You need to start out by fixing all the grounds, especially the instrument panel ground up under the dash. It should be a black wire that is grounded to a post on the emergency brake bracket. You will be amazed how many things in the cab are screwed up if that ground is disconnected.

It appears that someone added wiring, and in the process screwed up Multiple grounds and/or power wires. Then, instead of repairing it correctly they started adding new wires to fix individual problems with different devices.

FIX THE GROUNDS FIRST! Then disconnect obvious wiring additions and troubleshoot power sources.

 

[76 gmc k15]

Just wondering, but would a trailer plug in the rear bumper do this.

 

[76 gmc k15]

I would like to thank everybody for their help, it turns out it was the trailer plug it had a frayed wire that when it would touch metal it would cause all the weird lighting problems so I would like to thank everyone for there help.