Head light wiring issue on frankenstein Trans Am

[Sycostang67 78 2 Vehicles]

My friend had a 1974 Firebird, he bought a 1979 Trans Am shell and transferred everything over from the 1974.  A big difference is the 74 only had 2 headlights and the 79 has 4.  He had been using the 74 harness to run 2 of the 4 lights.  He bought a new 79 harness to try and run all 4 lights.  The plug at the firewall was different, but the wires were all colored the same so we spliced the old plug on the new harness.  We verified the same color wires went to the same lights on both harnesses as well.  Setup this way, only the marker lights and turn signals work, none of the headlights will come on.  There was power to all the lights(checked with test light, probably should have used voltmeter) and the grounds were good.  We put the 74 harness back on and everything still works that way, so we know the lights and switches all still work.  Open to thoughts/ideas, other things to check.

[Mopar1973Man 5,459 5 Vehicles]

Not good. Just because all the wire colors are the same doesn't mean they had the same purpose. You are going to have to trace all the wires one by one and their purpose. I've seen other brands and makes of vehicles do this where colors seem to be the same and wire a device up and find out the wire colors are all wrong. 

[Sycostang67 78 2 Vehicles]

We compared the harnesses side by side and the same color wires went to the same items on each one.  The headlights and marker lights used the same wires on each harness. 

[Mopar1973Man 5,459 5 Vehicles]

Kind of like when I replace a steering wheel in a Ford truck. Had all the same color wires. But, the colors went to different places year to year. I ended up pulling pins on the plug and sorting wire out going to all different places. 

 

You might have to break each connection and check for source signal and then find the proper lead to feed.