the one under the hood will get you every time Last time it happened to me I went through every light on my trailer before I thought about that fuse! glad you got it working.

I was getting ready to take it and pay someone to look at it. Glad I checked that 10 cent fuse.

I beg to differ. If the trailer is not properly grounded nothing will work. I assume that you are talking boat trailer. RVs are a different animal. Even on boat trailers the way the lights are grounded could be different. I have some marker lights that are grounded to the same place as my tail / brake lights.

On a system that uses the 1157 type bulb where the tail light filiment and stop light filament share a common ground inside the bulb(which is the most common system), if the tail light are activated with no ground the voltage will feed through the two filiments back to the filiments in the towing vehicle then to ground through them which will cause the lights to come on. When the brake lights are activated, 12 volts goes to the stop light filiment also and you have 12 volts on both filiments and the lights go out. LSUfan is correcto. bg

Glad you didn't do that! There is only a few things that can cause lights not to work. I've found the quickest way to narrow it down to either the vehicle or the trailer is to hookup to another trailer or hook another vehicle to your trailer. You'll learn real quick where to start looking

Shadetree is right, it is called back feeding.

back feeding doesn't happen with led lights though, seperate system all together due to the electronics in the light.