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How to Add Reverse Light Circuit to Trailer and Two Tow Vehicles


Hello: I have my tow vehicle wired with a 4 flat and 7 way. I have a trailer that uses a 4 flat that I would like to add reverse lights to but, I still want to have the 4 flat pin connection as another vehicle that may also tow the trailer only has a 4 flat. I have an extra 7 pin trailer connector and terminal box. I have two questions: 1.If I wire each of the 7 pins of female trailer plug to the 7 terminal block but, there isn’t a wire going to 12v power or trailer brakes will it result in an error on my brake controller since it would be looking for them? Should I leave those two not connected and capped in the terminal block? 2. Would it affect anything if I were to also wire the 4 flat to the 7 pin terminal block to have the 4 basic functions for the other tow vehicle? I understand I would need to have the 4 flat capped off when the 7 way option is place so power isn’t going out and open. I’m not a fan of the rubber 4 flat cover. I May take an old 4 flat connection, split the ends, strip and cap the wires. I hope I’m able to put into words and convey what I’m attempting. If yes, Are these scenarios even safe options? Thank you for your time


Expert Reply:

Thank you for all of the details!

It sounds like you have one vehicle with a 4-Way, one vehicle with a 4-Way/7-Way combo, and a trailer with a 4-Way. You want to add the reverse light function to your trailer using a junction box setup like what we have with part # e99011 and still be able to use the trailer with both vehicles.

I like the idea of adding the junction box as well as taking your trailer 4-Way and integrating it into the box so that all of your connections are made in one place and it will be easy to use. You shouldn't have to cap off any of the terminals since this would be a protected case. Your trailer brake controller might say that the trailer brakes aren't connected but that won't be harming anything.

Since one tow vehicle already has the 7-Way I recommend adding the Hopkins Adapter # 37185 to the vehicle that only has the 4-Way. This plugs into the current 4-Way and then you need to ground the white wire, connect the reverse light circuit, and then cap off the other circuits. This gives you a permanently-mounted 7-Way connection compatible with your trailer connector so you don't have to keep track of an adapter that you plug in every time. A no-drill mount bracket like part # C57202 gives you a way to mount this connector near your trailer hitch receiver for easy access.

If I misinterpreted anything please let me know!

expert reply by:
Jon G

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