Trailer Running lights not working
 

Folks, I'm having a problem with the running lights on my trailer when i tow it with my blazer. It works fine on my Dad's jeep and everything else (signal, brakes, Hazards) work fine on my truck except the running lights. I checked all the fuses and they seem fine. The truck running lights work fine too when the the trailer is connected it is just the trailer not working. I cleaned up the connectors too. Is there anything else I can try? It isn't' a big deal during the day but its pretty dangerous at night with a black trailer and no running lights on it.

When you say you checked the fuses, did you check the ones in the underhood box? The trailer wiring has its own fuses. Another possibility is a former owner replaced the plug with the "twisted wires & black tape" approach. That comes back to haunt you eventually. Check the harness on the back

Trailer lights
 

If all the trailer lights are out and the fuse is OK, check the ground. Measure voltage to assure that there's 12v at the truck connector. That's what caused mine to fail (bad ground).

I bought the truck new and had the harness and hitch professionally installed. I did check the fuses under the hood. They are ok. I haven't put the meter on it yet so I will try that.

I had a similar problem and determined it was a bad ground. I ran a separate line bolted to the trailer frame that runs to a bolt/wing nut on the rear metal bumper and that solved the problem. To test for a poor ground just temporarily ground the trailer to the car by using a set of jumper cables and see if that helps.

How exactly do you ground the car with the jumper cables? Where should the ends by connected?

Most of the time, the ground is through the ball into the hitch on the trailer. If you wanted to add a supplemental ground (not a bad idea), you would install a wire onto the frame of the trailer and find another suitable location for the lug on the truck so that you can easily connect/disconnect the ground when you drop your trailer.

i was just gonna say that Kyle. i've never had an issue wih the ground on any of my trailer-hell, i had the ground wire ripped off one for over a year, no problems.

take apart the plug (connector) on your truck. i don't remember if you mentioned what year your truck was (and i can't see any sigs when replying, so....), and i'm not sure where you're from (another good reason to put that info in your profile) but here in Ontario, i usually change them every other year, and at the very least, pull it apart to inspect before each Winter.

*edit* ahh, just went to check and see if you info was in your OP, and it's not, so add this info for future help, 'cause it does.

The box that is installed as part of the Harness so the Bulb Out light on the dash works normally and also makes the trailer lights work with the Third Brakelight on the Blazer sometimes fails.
Happened on my brothers Jeep. Some lighting functions worked while others didn't.

first Tony, what "box" are you talking about? 'cause you're saying this could be the problem, and give proof by the way of your brother's Jeep.

not sure what that may be, but a Jeep ain't no Blazer. and is that "box" thatyou refered to, is a factory set-up? 'case he mentioned that "I bought the truck new and had the harness and hitch professionally installed.", and buy that i would take it was not @ the dealer. but even if it was, it's still not a factory towing set-up.