Trailer lights issue: Lights wont turn off?

Ok guys, for starters, I've been through my fair share of trailer wiring. I know to check the grounds, and I've even re-wired trailers a couple of times, so I feel like I've got a pretty good baseline, but this one has me stumped.

I have a 2 axle car hauler (not a boat trailer, I know, but a trailer none the less...) with brakes on one axle. It is all wired to a 7-way plug, which we plug straight into the factory outlet on our Excursion. Now, we know we have a ground problem on the vehicle, but I can't find it. Also, both the vehicle and the trailer have 4 wire plugs, so we just plug those up also and everything works fine. Or at least it did till a few weeks ago...

Here is the problem: When I hook up the trailer and turn on the lights, everything is fine. When I leave the trailer hooked up, turn off the lights, and turn off the car, all of the parking lights stay on, both the truck and trailer. However, if I unplug the 4-wire harness, all the lights go out, and stay out until I turn the headlights (or parking lights) back on with the switch.

I'm stumped. Any ideas?

Re: Trailer lights issue: Lights wont turn off?

Since you have electric brakes you should also have a 12vdc source charging a battery type device for your breakaway circuit, I am wondering if you have it crossed over at some point????
And it is feeding back through.

Re: Trailer lights issue: Lights wont turn off?

Thanks rbh, but Alabama doesn't actually require a breakaway device on trailers, and this trailer doesn't have one installed, so no battery.

Re: Trailer lights issue: Lights wont turn off?

OK. I am a little confused by your post. Are you plugging it in to both a 7 pin and a four pin? If you only use the four pin, then your brakes do not work? Why would you need the four pin if you have the seven plug in place?

Re: Trailer lights issue: Lights wont turn off?

I only use the four pin as a secondary ground because I know the ground in the 7 wire is goofy and I can't find it to fix it.

Re: Trailer lights issue: Lights wont turn off?

The thing with the lights staying on I believe is you have one of the wires constant with 12vdc and it is back feeding through your markerlight circuit, thats why when you pull the plug the lights on the truck and trailer go out.

Re: Trailer lights issue: Lights wont turn off?

That actually makes a lot of sense, and I'm kinda ashamed I didn't think of it. When I wired the plug, I hooked up that wire "in case I ever need it..." Thanks RBH, I'll just pop the plug open and unhook that wire to see if it helps!

Re: Trailer lights issue: Lights wont turn off?

I agree on the double feed. I would also suggest you do what ever is necessary to figure out the ground issue on the 7 pin. I believe the ground is necessary for the brakes and if it fails you have no trailer brakes. It may be in either the trailer or vehicle 7 pin connector.