3rd brake light connection - already wired for a topper

My truck previously was wired to hook up to a Leer topper 3rd brake light, but I can’t figure out how to connect to the GFC provided connection - 1st photo is the connection from the truck, and 2nd is from the GFC. Can anyone help?

From what I was told at the install, you’d clip the connectors on the GFC end and replace them to snap into the other wiring harness. I have the same thing going on - it’ll be a few days before I get to this but will post up if you haven’t gotten it done before I do.

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Hard to know without a multi-meter, but I’d go black to solid black and red to solid red. I’d guess the other wires in the leer harness are dome lights.

The GFC supplied connectors have a needle you will see if you unscrew the end. You can just spear them into the wires up from the connector.

I’d personally probably buy another 4 pin connector are put the GFC wires into it to keep things clean.


As @jedgar said, you’ll need a multimeter to be sure. I wouldn’t trust assumptions on what is what. Depending on how it was wired out it may be set up to also be tied to signals (considering it’s 4 pins). In my situation the topper wire out I got from Cap-it when I had my cap installed is, 12V constant (dome light), Brake light, Ground. 4 could be Ground, L signal, R signal, Brakes…could be Ground, 12V, Brakes, and nothing else…only a Meter can tell you. If you’re unfamiliar with using these tools I could suggest you probably have a pal close by with that knowledge to help you out.

The last thing you want to do is be blowing fuses, or worst case burning your truck down.

Great thank you for the info!