I just had electric brakes ($160.00) hooked up to haul a large camping trailer. Everything worked great. Now that I want to hook up the boat trailer I only get L and R turn signals, brake lights and hazards if my headlight switch is in the OFF position. Which means no trailer running lights after dark. Running lights did work before the new brake system was installed. I also simulated darkness by covering the sensor on the dash and the trailer light system acts as if I turned on the headlight switch.
I have the seven wire hookup on the back of the truck and the boat lights all worked before the electric brakes got hooked up. The auto shop hooked up their simulated light system to the back of the truck and all seemed to work properly, but as soon as I hook up my boat (5 wire flat) plugin, it will now only work if I keep the headlight switch off. Any ideas on what is going on with this? I also checked every fuse I could locate and no blown fuses. Could not check relay switches, but none are listed that I can see that have anything to do with the light system. The auto shop said that there is some corosion in the plugin on the back of the truck that could be causing the lights to act that way, but like I mentioned earlier, their simulated light system that they plugged into the plug-in on the back of my truck, worked okay. Any ideas on why it is not working for me? (Plug-ins on boat trailer are new also.)

 

Check your grounds on the boat trailer. A weak ground may not be able to handle all the running lights on at one time. I have had all kind of weird trailer lighting issues brought to me and a good percentage of the time it is ground related. Many times I clean the main ground and run another to the back of the trailer to each light as well. If the other trailer worked right I think that isolates it to your boat trailer. Good luck!