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I’m very new to C3s, I owned a c4 beforehand. My new one is a 1975 coupe with the L48 and TH400 transmission. I’m having some trouble with the running lights not coming on when the headlights are engaged, but the brake lights work when the brake is applied. I was looking underneath to check the grounds and I found this red wire just sitting out. I googled what the red wires were and google told me they were brake light wiring, but my brake lights work flawlessly. I was wondering if anyone could clue me in to what that connector might go to or some possibilities of what the issue could be
Last edited by brightred85; Feb 23, 2026 at 09:10 PM.
Where are you looking when you took the picture? I looked at a B&W wiring diagram and don't see any Red wires in the back. It might be for the alarm horn, but those wires are supposedly Pink, but it is two wires in one connector. The Alarm Horn was mounted behind the left rear tire in earlier years.
I am including a 1975 wiring diagram. I don't know who made it, but it isn't a GM diagram. I was in two pieces, but I put them together. I have redrawn a number of earlier diagrams into color, but I have stopped at 1969.
Where are you looking when you took the picture? I looked at a B&W wiring diagram and don't see any Red wires in the back. It might be for the alarm horn, but those wires are supposedly Pink, but it is two wires in one connector. The Alarm Horn was mounted behind the left rear tire in earlier years.
I am including a 1975 wiring diagram. I don't know who made it, but it isn't a GM diagram. I was in two pieces, but I put them together. I have redrawn a number of earlier diagrams into color, but I have stopped at 1969.
Behind the rear wheel, drivers side is alarm horn. Nothing whatsoever to do with your lighting.
perhaps start at the switch, do you have power coming out of the switch? If so start moving in the direction of the lights. Power through the firewall connection, etc.
Some wiring diagrams may be found at the link below (if it still works). The running lights are brown wires.
Do you have any LED bulbs? You can absolutely run with 100% LED bulbs if you want, but they sometimes require some modifications to work. You can switch to incandescents for debugging.
Behind the rear wheel, drivers side is alarm horn. Nothing whatsoever to do with your lighting.
perhaps start at the switch, do you have power coming out of the switch? If so start moving in the direction of the lights. Power through the firewall connection, etc.
The unattached wires you are referring to should be connected to your alarm horn. Your alarm horn, if still there, is mounted above your driver's side muffler. Clean off the wires and you will see they are pink, not red. See the wiring diagram from the 1975 GM Service and Overhaul Manual Supplement below:
There are no red or pink wires that connect to your rear lighting. Keep checking your ground wires. You are on the right path.
Invest in this book published by General Motors. It has the most accurate (but not 100% accurate) wiring diagrams for your '75. All other wiring diagrams are taken from this book.
Good luck!
Last edited by '73and'75; Feb 24, 2026 at 04:56 PM.
The unattached wires you are referring to should be connected to your alarm horn. Your alarm horn, if still there, is mounted above your driver's side muffler. Clean off the wires and you will see they are pink, not red. See the wiring diagram from the 1975 GM Service and Overhaul Manual Supplement below:
There are no red or pink wires that connect to your rear lighting. Keep checking your ground wires. You are on the right path.
Invest in this book published by General Motors. It has the most accurate (but not 100% accurate) wiring diagrams for your '75. All other wiring diagrams are taken from this book.
Wiring help
