Running/Headlights cause trailer lights to not work….

I couldn’t find a thread with this issue addressed, and so im sorry to start another trailer wiring issues thread but here it goes..Purchased a boat YAHOOO….trailer lights on it worked but were in poor condition cracked, old wires, gobs of tape, etc. and so I assumed issues were related to those things(as far as I know issues were the same as below with old wiring). So I purchased a brand new trailer lights kit from Walmart, Attwood brand kit, and spent the morning installing the lights. Everything is installed but here are the issues.

1. When the running or headlights on the truck (2002 Ford Ranger) are not activated, the trailer will function pretty much normal except when using the left(driver) turn signal it also faintly causes the right(passenger) turn signal to activate, so that’s issue one.
2.Issue two is when the running or headlights are switched on, the brake light on left(driver) side activates and the turn signal does not work on TRAILER OR TRUCK, but right(passenger) side turn signal works correctly both on truck and trailer.

I have not noticed any issues with any other trailer I have towed with vehicle. I did not use the included truck wiring harness from the kit as I already had one installed, the trailer does have two small running/indicator lights on sides, that I hooked up exactly the way they were with old wiring, however only the drivers side light works. Any advice from those who have had this headache before?

First make sure all of your grounds are clean, free of corrosion, and tight on both the trailer and vehicle.

Definitely a ground problem. Start with the truck ground and work back through the trailer. If you have a tilt trailer add a ground wire from the front of the trailer to the back half of the trailer where it pivots. Make sure where all your lights bolt on that you have shiny clean metal where the nuts tighten.

Resolved the issue…I checked the ground and that was not an issue, turns out the way the side indicator/running lights on the trailer were wired was causing some sort of short or connection issue, I basically disconnected them and now everything is working correctly. I don’t believe the trailer requires the amber side lights, as its only fitting a 14ft boat on it I don’t think its long enough.

if the trailer at it’s widest point is over 80″ wide, the side marker lights are required.

I’ll measure for sure but I doubt the trailer is over 80″ wide, boat is only 48 on bottom, but ill check then look up how to wire in the amber lights correctly as the previous method (same as when I purchased trailer) clearly didn’t work.