What Trailer Wiring do I Need to Put a 4 Flat on the Back of My 2025 Hyundai Tucson?

Updated 09/02/2025 | Published 04/28/2025

Question:

Im trying to locate a trailer wiring harness for a 2025 Hyundai Tucson.

asked by: Derick W

Expert Reply:

Hello Derick,

Congrats on the new ride! What is your favorite new feature?

For your 2025 Hyundai Tucson you will want the ZCI Circuit Protected Vehicle Wiring Harness w/ 4-Pole Flat Trailer Connector and Installation Kit # 119250KIT. Hyundai changed the electrical system plugs so there is not a plug-n-play option at this time. Takes a while for the aftermarket guys to get caught up when they make big design changes like this from the 2024 to the 2025.

expert reply by: Kate F

Joe T.

8/29/2025

Hi Kate I have a 2025 Tucson. I installed the Tekonsha 119251. Everything works but I have a constant 12v at the brown trailer tail lights. I tried everything to solve this even replaced with another unit but can't figure it out. Any suggestions. Thanks. Joe T

Kate F.

9/2/2025

@JoeT On the 2025 Hyundai Tucson, the lighting system is a little tricky. Hyundai uses “smart” wiring where the same wire can carry multiple signals, or it sends pulsed voltage instead of a steady 12V. The Tekonsha ZCI sometimes reads that as “always on,” which is why your trailer tail lights are staying powered. A common culprit is the DRL (daytime running light) circuit. On the Tucson, the DRLs often overlap with the tail light wiring, so if the ZCI clamp is on that wire, it’ll keep the brown trailer circuit hot all the time. The first thing to try is moving the brown ZCI clamp. If it’s on the tail/parking light feed, try the other side of the vehicle, or even the license plate light wire, that’s often a “cleaner” signal the module can understand better. When your Tucson is off, are your trailer lights fully bright or just a dim glow?

Joe T.

9/2/2025

@KateF Fully bright. I noticed that when the car is powered off the trailer lights stay illuminated. The trailer lights would go off for a minute then back on. I removed the brown trailer connector and I still get trailer lights.

Kate F.

9/2/2025

@JoeT Since your trailer lights are staying fully bright even when the Tucson is off, and they go off for a bit then come back on, that tells us the ZCI box is holding the brown circuit “on.” The fact that you removed the brown sensor and the trailer lights still lit up really seals it, the module is the one powering them, not the sensor choice. What’s happening is the Tucson’s computer (BCM) keeps sending little “pings” or monitoring pulses through the lighting wires, even when the car is off. The ZCI is super sensitive and thinks those pings are a real signal, so it switches the trailer tail light circuit on and off. A couple of things you can try: - Do a reset by pulling the ZCI’s fuse or disconnecting its battery lead for a few minutes. Then hook it back up and let it relearn the signal. - Try clamping the brown sensor onto the license plate light wire instead of the main tail light wire, it often has a cleaner signal the ZCI likes better. - Double-check that the ZCI’s 12V power feed is coming straight from the battery. If it’s tied into another always-hot circuit, that can make the problem worse. I know they changed the wiring system between the 2024 to 2025 which is why there is not a "plug-n-play" option at this time.