South East Toyota wiring harness trouble.

My '14 Tacoma (4cyl 5 lug 2wd 5spd) did not come wired for a trailer. I installed my own Draw Tite, and purchased the "Southeast Toyota" version of the plug -n- play wiring kit, with connections to the battery, ground, and plug ins at the driver and passenger side tail light. There is a small black box (relay box?) that gets mounted in the driver tail light housing. Long story short, I hooked up to a trailer today (with LED lights) and got NOTHING, my neighbor (an electrician) came over and tested it, said everything looked okay on a multimeter, and I tested it with an incandescent bulb from my mower, worked great... Is this an LED thing? Anyone else using this same harness from SE Toyota without trouble? Any help would be appreciated, I have a call out to the dealership but have not gotten a call back yet.. Thanks for the help!
.... I posted this in "towing" and thought maybe I could find some help in "lighting"

 

Have you ever seen the lights on the trailer work with another vehicle attached? My first thought is reversed polarity at the trailer lights. LEDs only work with correct polarity.

 

Yes, the worked on an F150 which was sitting right next to my truck. I talked with people at the parts AND service department, neither of them were of ANY assistance, nor had any idea why their part was not working, but they also couldn't say if they had any other reported problems with LED lights, or even if it's design would prevent it from working with LED's. Thanks!

 

If you're getting power at the connector on the vehicle side, then it's the trailer. Probably a ground issue. Sometimes they will ground through the ball.

I have LEDs on my trailer and they work fine.

 

I didn't have it hooked to either truck, and they worked fine with the F150 "shop truck" could it be a difference in how MY truck is grounded?
Thank you for your help again!

 

The black box converts the amber turn signals on your truck to work with the trailer brake lights as turn signals.

I would check the resistance of the ground terminal on the vehicle side trailer plug.
Hook one lead of the ohm meter up to the exposed terminal on the plug and the other lead to the vehicle chassis. Should be close to 0 ohms.

 

It's got to have something to do with the LED lights not pulling enough power. I can't wrap my head around why right now, but whatever is in the convertor box is going to be solid state electronics, it must need a certain amount of current draw to work properly.

Is the part a Toyota branded accessory? If so, I would go down to NAPA and buy a Reese or Hopkins brand harness and try that. I sold a lot of them when I worked in auto parts and never had anyone bring one back.

Edit: Before that, double check all your grounds. If the lighting kit for the truck came with a loose ground wire that needed to be connected with a screw, pay real close attention to that, I put some extra lights under my trailer hitch and it was very difficult to get a good ground back there. Had to grind the paint off my factory hitch and get it really clean. I also have had problems in the past grounding to truck beds, they are usually mounted using rubber isolators, although I haven't looked at the Tacoma real closely.

 

I have a small trailer that I use to haul a single motorcycle. I've wired it for running and brake lights only because the "box" kept letting me down. The blinkers on the truck are plenty visible in my application. I'm not sure what you're towing.