I have a problem with the electronics on my Blazer. Park lights are ok, but with headlights turned on, both hi-beam and the left direction lamp (canstantly) is being turned on. And turning them off is impossible, except turning off headlight. But that is not an option. I have tried everything. Both headlights i have taken off. Cleaned with isopropanol. Checked earth.
and the visual wiring.

 

Do you have a DMM (digital multimeter)? If you do, I would check for continuity between the power wires for the directional lamp and the highbeam headlight. If you get a tone, then the wires are crossed somewhere down the line. They may be crossed elsewhere as well.

What year of truck are we talking about here? It's possible with age that you may have some cracked wires in the harness that are now contacting each other and supplying power to circuits that they were not originally intended. Also, if you have ever had rodent problems, they can make a mess of wiring real quick!

 

Yeah exactly what he said, in auto we had to work on this old rabbit and a bunch of the wires had parts of them that had worn down and didnt have any insulation left. A bunch of them contacted on other wires and it was a huge mess

 

This is the stuff that I live for guys! I love finding those electrical gremlins!!!

It sounds like you have voltage backfeeding through your parklamp bulb circuit in search of a better groung. Check your headlamp grounds first...If you don't know where they are just run a redundant (additional) ground wire from each or the headlamp connectors to ground.