Chrysler Repair: 2012 kia sorento with after market curt hitch and sorento wiring harness 4 flat, kia sorento, vehicle lighting

Question
QUESTION: Hi Roland.

A year ago, I installed a curt class III hitch with a sorento specific wiring harness.  It worked perfectly UNTIL, I purchased a horse trailer that was rewired by the previous owner.  It was a crude job, but it worked for him as I witnessed him towing the trailer.  Trailer running lights, brake lights and turn signals worked perfectly for him.  He had a 6 round connector on the trailer.  I was wired 4 flat, so he spliced the wired and put a 4 flat trailer end on.  Ok fine.  

As soon as I plugged the trailer into my vehicle hitch wiring harness, I noticed that the trailer running lights illuminated.  I did not have my vehicle lights on.  Regardless if I turned my vehicle switch on or off, the trailer running lights illuminated and the turn signals both right and left on the trailer did not work.

Here is the problem:  once I arrived home, not long after,  the left trailer light did not work at all, and the entire left side of my sorento's lighting (except mirror turn signal) didn't work.  Took it to dealer and we figured out that when the left tow harnes wire was disconnected, the vehicle lighting worked again.  If connected, vehicle lights don't work.  I am re wiring the trailer and converted it to led.  I have yet to figure out why my perfectly good vehicle hitch wiring is now faulty and what do I do to fix this.  No one seems to be able to help.  

Thanks

ANSWER: Hi Karen,
I have to beg ignorance of the cause of this electrical problem because I have neither the wiring diagrams for the Kia of for the trailer. If I had them that would possibly be a different story, but it is really difficult to analyze this sort of a problem without any wiring diagrams. Because it is one-sided, perhaps the best approach would be to compare the left and right sides of the trailer's wiring for any difference that could explain why.
Please 'rate' my answer (see the PS below).
Thanks,
Roland

---------- FOLLOW-UP ----------

QUESTION: Hi Roland.  

Yesterday, I pulled all the wiring and lights from the trailer.  ALL OF IT.  He had splices all over the place, way to much wire slack and I wasn't going through that.  Since I am converting trailer LED, I wanted to start off with a proper and neat wiring job.  My question actually has nothing to do with the trailer, only why my left side lights won't work when the tow harness is connected to my left tail light, and why when that is disconnected, do they work?

I already know root of issue was the trailer.  However, I am still stuck with a vehicle harness issue.  Since I have disconnected the hitch wiring, the problem is there. I'm assuming.  The car lights work, only when aftermarket wires are disconnected.  This leads me to believe the problem is in the hitch wiring?????  I have already spent a lot of money at the dealer and a trailer shop. I'm hoping all I will need to do is re do the left wire on the vehicle side 4 flat???  I am not comfortable messing with my car's electrical.  

Trailer wiring diagram is standard for 2 horse bumper pull.  It was wired for over 80".  Left turn on yellow, right on green, brown running lights, blue brake, white ground, grounded to frame.  Wiring was correct, but sloppy.  Something on the left side caused a short ? because both left and right brake/running turn didn't work properly from the beginning, then not at all an hour later.     

An educated guess would be fine for me.  I just need a direction to start.  I will compare once I finish wiring the trailer.

Thanks for trying to give an answer, at least.
Karen

Answer
Hi Karen,
I would suggest that you install the right side wiring first and verify that the LED's work properly and that the vehicle lights work properly when that side is completed. Then do the left side identically and ideally it should function the same as the right. If not, then you would be helped to have an ohm-meter to check for short circuited wiring on the left side of the trailer wiring and compare the resistance readings to ground (any chassis metal) of each of the 4 wires, to those on the right side. If the readings are the same then I would believe that there is mis-wiring related to the right side of the vehicle's wiring or for some reason the circuits for the rear wiring are not identical on the left and right sides. The latter would be best understood if you had the Kia wiring diagrams. But you should be able to examine the plug that is attached to the right and the left side wiring of the vehicle to be sure that the wires going to each of those plugs is attached identically on both sides as far as color-to-color of wire is concerned.
If the wire in the vehicle are identical on both sides, and the plug that attach to those wires are identically connected then in theory the two sides should behave identically when attached to properly installed trailer wiring.
If the problem still exists at that point:
See if you can get a circuit for the 'rear lighting' of the vehicle. I have heard the the Autozone.com web site does have such diagrams available at no cost.
Thanks for the nomination and rating. You can do it again in response to this answer which I hope will be helpful to you.

Roland