Flashing Brake Light When Towing a Trailer With a 2024 Toyota Grand Highlander
Updated 02/10/2026 | Published 01/19/2026
Question:
I purchased your T-One TK99FR for my Toyota Grand Highlander. Plug and play installation went simply. But when I finally got to test the wiring this week I had an issue. Both with the trailer and also I purchased one of the cheap 4 pin testers and got the same results: Left turn on car signals right on the trailer. Right turn signals left. I have switched the yellow and green wires which corrects that. But also when I press the brake pedal, the brake lights flash but do not stay on even when the pedal stays pressed. Searching on the internet indicates others have the same/similar propblem with aftermarket converters but not with the official Toyota Denso units? Someone said their turn signals flip when teh brakes are applied? There is rumour of a Toyota TSB on the issue?
asked by: Nigel H
Expert Reply:
Hey, Nigel.
That’s a great question. Is the flashing consistent, or does it seem erratic and then stop? When you were using the tester, did the brake signal flash as well, or does it only happen when the trailer is plugged in?
A bad ground can cause all kinds of wiring issues and is usually the most common cause in situations like this. Just to confirm, is your 2024 Toyota Grand Highlander equipped with or without a tow package? Having a tow package doesn’t necessarily mean it already has wiring, but it could be prepped for a simpler plug-in harness.
I’m not quite sure what you mean by “someone said their turn signal flips when the brakes are applied.” The harness combines those functions so that on the trailer, the same light is used for braking and turn signals. Even when braking, the turn signal should continue to blink.
Let me know, and I can go from there!
(click to enlarge)
Continue Researching
- Q&A: What is the Difference Between a 4-Way and 7-Way Trailer Connector
- Article: Brake Controller Installation: Starting from Scratch
- Article: Brake Controller 7- and 4-Way Installation Kit (ETBC7)
- Q&A: What is Difference Between Redarc Tow Pro Liberty and Elite
- Q&A: What is the Difference Between the Tekonsha Prodigy P2 and the P3 Brake Controllers
Gwendolyn Y.
2/10/2026
Hey Nigel, what ended up happening with the T-One wiring harness on your Grand Highlander after checking the ground and tow package setup? Once you sorted that out, did the brake lights behave normally? Your update could make it easier for others running into the same issue.