Why are your trailer brake lights staying on when you turn your headlights on?

Answer 1

First you need to understand that most, if not all, vehicle [autos, trucks, and trailers] rear lighting systems have both tail light bulbs and brake light bulbs, OR a single bulb on each side [right and left] that each have two filaments [what glows when electricity is applied]. The tail lights are supposed to be on when the headlights, or parking lights, in the towing vehicle are turned on. The separate brake light bulb, or brake light filament in the dual filament bulbs is only supposed to turn on when the brake pedal is depressed, and the towing vehicle brake lights are on. I suspect that what you are calling brake lights are actually the tail lights. To check this out, run some tests. Have an assistant sit in the vehicle and do the following on your command: 1. Apply brakes by pressing on the brake pedal. The brake lights should turn on. 2. With foot OFF of the brake, turn on the headlights. The trailer tail lights should turn on. 3. With the headlights on, have the assistant depress the break pedal. Even though the tail lights are already on, the brake lights should come on also, causing the trailer lights to brighten noticeably 4. While you're checking, have the assistant turn the ignition key to on, and one at a time test the left and right turn signals. Again, I think what you think are brake lights, are actually the tail ligts.