I just recently bought a trailer kit. It's one of those that you have to assemble yourself. I got it mostly assembled and got the lights hooked up. That's when the problem surfaced. I tested the lights by hooking them up to a truck that I had towed other trailers with before. Everything worked fine until I turned on the headlights. When I turned on the blinkers, brakes, and hazard lights, the lights lit up just fine, but then when I turned on the headlights, the lights stopped working. They turned off. No more blinking for turn signals, hazard lights or brakes. But then when I turned off the headlights again, the turn signals, hazard lights, and brake lights worked on the trailer.

Any ideas? Has anyone seen something similar before?

I'm not exactly an electrician, so make sure you enunciate and spell everything out really, really slowly.

Okay I have a few ideas.

Now my dad took 2 years I think to be a electrician but dropped out because of all the "fake work" they kept giving him.

We have been through a mess of stuff like this on the farm with trailer brakes and such.

You first need to get a power checker. You can get it at walmart in the AUTO section. It looks like a screwdriver with a light in it.

You can test what wires are HOT when the lights are on and when they are off.

You could be on a HOT wire when the lights are off and not when they are on as they get re-routed.

Once you figure out what wire is HOT while the lights are on hook onto it.

Another idea could be you are drawing too much power from that particular wire and need to find another wire so step 1 could solve the problem.

Hope this helps.

Make sure you have a solid ground between the trailer and the vehicle frame. A faulty ground can cause all kinds of strange problems.

I'll confirm what ddeerrff just said--sounds like your truck is wired correctly and an improper ground will cause some problems like that that don't make a lot of sense. If you've tried grounding to a painted surface, try using a fastening bolt where you have good metal on metal contact that is clear of any paint or rust.

Here's an update on what I did today:

I connected the yellow and green wires to the front marker lights, and scraped some paint off the spot where the ground wire screws into a "self-tapping screw". The way I had it before was the way the instructions wanted me to set it up, but that didn't work, so I changed it. The way the instructions had me do it was to connect the front two lights to the brown wires. That didn't work yesterday so today, I connected the front two lights to the yellow and green wires.

Here's what happened. The front lights no longer worked as turn signals, but all the lights still stopped working when I turned the vehicle headlights on.

When you buy trailer wiring harness connector Plugs here,it comes with a colour coded wiring diagram in the box.
If its the same in the US,why not got to an auto shop and ask them to photo copy the wiring diagram for you.
Cost you nothing, and you will know what colour wires go to what.Hope this helps.

EDIT.If the tow vehicle is wired correctly to the Plug,you shouldn't have a problem.
I used to wire up many trailers in my workshop, and seldom had a problem going by the diagram supplied.

Plan B.This may help many of you that try and fix these things at home.
Go to an Auto or Auto Parts Shop .Buy yourself a cheap wiring test light.About 3 bucks here.
EG:turn on park lights.Stick the testlight probe into each wire socket on the car.When you find which wire has power,lights up the testlamp,trace the park light wire from the trailer and join them.Providing the ground connection is clean,and you have the correct trailer wire to ground,and the globes all OK,then it MUST WORK.
Continue with all lights the very same method described and your wiring headaches will be over.

EDIT.With this method,it doesn't matter what colours the wires are.

I'm thinking there must be a bad part in the trailer somewhere. Probably one of the taillights or the actual connector plug, since the wrong cables seem to be doing the wrong things.

I've used the vehicle to tow other trailers before and the lights on the other trailers always worked right.

The wiring diagram looks like this one (the four-way system) but doesn't work like it's supposed to: http://www.accessconnect.com/trailer_wiring_diagram.htm

I have a tool called a "circuit tester" but haven't the faintest idea how to use it to isolate the bad part.

WE have the 6 and 7 pin Pin Plugs as shown in your diagram.The fuse for the lights sounds OK,so you haven't got a short circuit.Doubt if you have a clean break in any of the wires.These 6 and 7 pin plugs here can be pulled apart and cleaned.If yours cant,no problem.Power from the car enters the socket on one side,test with the test light on the opposite side that power is indeed getting through the socket.Just poke the sharp end of the test light staight into the wire through the plastic coating.Do this with all connections coming out of the socket,providing power is going in the opposite side.If its a sealed plug and power is not getting through,you have no option but to buy a new plug.
Check to see if the test light is working properly first on the battery poles.Test lamp ground to NEG on the Battery,probe on test light to POS on the battery.I have tried to explain as simply as I can.
On every test the ground on the test light has to be grounded to a clean metal surface,to work.
If for example you turn on the indicator and there is no power coming out of the car socket,you have to trace those wires to where they are attached to the car electrical harness.

EDIT.If you say which I just read that this trailer works OK on other vehicles,then the wires are incorrectly wired to the car socket side or the socket is faulty.IMO

I don't think I said that, at least I hope I didn't say that, since it would have been incorrect. I've used this vehicle to tow other trailers before and the lights on the other trailers always worked right. But I've never had this trailer working right and have never used this trailer on any other vehicle.

You are right,Then disregard my edit post in #9. I am always sleep deprived,and sometimes I misread a line.I blame BC for that.
Read my other posts carefully, and you should be able to sort it out.

Try replacing the flasher with a heavy duty unit.

Go back through your trailer wiring

White - Ground
Brown - Tail and marker Lights (cut the brown wire and use a 3 way splice to avoid lighing issues ie incoming Brown to side marker light and then an outgoing brown to the rear tail lights)
Green - Right Turn/Stop
Yellow - Left Turn/Stop

Ground the white wire to the frame and then ground all lights separately to the frame

Check your truck/car all of the incoming power should be coming off of the taillights and wired to match

Towbar, Trailer & Caravan 12N Wiring Diagram

Here's some more food for thought ...
If the only lights on the trailer are tail lights and they are wired properly then there is a faulty tail light shorting to ground.

Try disconnecting one tail light at a time and see if the rest of the lights work properly.