Trouble Finding Brake Light Wire on a 2025 Ford F-150

Updated 05/19/2025 | Published 05/16/2025

Products Featured in This Question

Roadmaster Brake-Lite Relay Kit for Towed Vehicles

(762)

$52.02

Roadmaster Stop Light Switch Kit

(696)

$43.65

RoadMaster Stop Light Switch Kit

(696)

$47.85

Question:

I am trying to install the RM88400 on my 2025 Ford F-150 Lariat. It has the push button ignition and the brake lights do come on when I push the brake pedal with the ignition off. I assume I need the B installation instructions. I cannot seem to locate the actual brake light switch to be able to install the relay? Any ideas on how to locate it?

asked by: Dennis D

Expert Reply:

Hey Dennis, thanks for reaching out!

According to the Roadmaster fitguide you're going to need a brake light switch as well since they weren't able to locate the color of the OEM brake light circuit wire. You'll use part # RM42MR if you have an adjustable brake pedal, or part # RM89ZR if you don't have an adjustable brake pedal.

Once the brake light switch kit is in place, you'll have the signal you need to install the Brake-Lite Relay Kit # RM-88400.

Are you installing all of your flat tow parts by yourself?

expert reply by: Jon G

Dennis

5/19/2025

Jon So I still don't understand how this will all tie together. So assuming I install the RM42MR using it in place of the OEM switch I am still a little confused on how this works with the truck lights. Specifically coming off of Pin 30 (the black wire) of the RM-88400 relay where would I need to run the wire to?, back to the rear tail lights connecting in place of the Brown tail light wiring? And yes, I am doing the install myself. So far this is the fourth vehicle 2017 Ford F-150, 2020 Lincoln Nautilus, 2024 Ford Edge ST (don't ask) and now the 2025 Ford F-150 Hybrid. (this was the easiest install of the baseplate so far.

Jon G.

5/19/2025

@Dennis Good question (and I'm glad you're a seasoned DIYer!). The stop light switch is going to be on the "input" side of the brake light relay. So if you're looking at the "Normally Open" diagram the brake light switch will need to attach to the butt connector for the green wire that connects to "87", and for the "Normally Closed" diagram it will be attached to the butt connector for the blue wire that connects to "87A". Then the black "30" wire from the output on the relay kit will connect to the brake light wire on your F-150. You will follow the rightmost part of the diagram with that brown wire and the # RM-790 to tie the signal into your wiring kit as well. Hopefully this clears things up for you a bit - let me know if you have additional questions!