trailer lights wont work

You defiantly have a short somewhere, take a multi meter and make sure that your outlets on you truck match the plug on the trailer, it sounds as if there is a difference to me, thus the lights going off in a 1/2 second.

One way you can check to make sure the trailer marker lights are working an not shorted is to put a jumper wire with a small amp fuse (5 amp) between the battery and the marker lights on the trailer pigtail, they should light and stay on, if they don't then the problem is in the trailer and not your truck .... You can do the same with the brakes but you need someone close to the brake drum to hear if they are applying, it may take a a bigger fuse to do this ... I can say I never have put any protection type device in line in any of my tow vehicles, I've always let the vehicles fuses do the work that's what they made for to protect your wiring :) ...

good luck. let use know how it turns out

Make sure the ground wire and connection between the car and trailer is clean and secure.

Im hoping its a weak ground. I know the trailer lights work perfectly, as I have simply hooked my truck up to it to verify. All wires in the plug are perfectly matched to the trailer plug harness.

I looked in the mirror a few months ago and saw I didn't have any trailer lights. After checking things out I checked the fuse and sure enough it was blown. Bought a new fuse installed it and I had lights again. Don't know why it was blown but they have been on ever since.

Little late to the party, but did you get your electrical issue sorted out?

I had similar problem, and while I wasn't able to troubleshoot, I dropped the vehicle off at a shop. Turns out that a junction box of some sort was blown ($198 box...) probably when at Kia for regular maintenance. I got power to the trailer, but no running/brake/turn signal lights. My wife picked up the car just today so I don't have all the details yet.