2006 3500 back up lights inop

5 pin trailer connections here went out when Noah died. They were standardised at 7 pin for a long time then for a short time there was 7 + 1 and even that didn't provide for reverse lights.

Some 30-odd years ago there was a return to a single 7 pin for trailers with an additional 7 pin for caravans, the two designated 7N and 7S and the supplementary plug provided facility for caravan battery charging, a 12 volt supply, additional grounds and reverse light.
Today the standard Euro 13 pin system is creeping in.

Although it is impossible to buy a 5 pin trailer plug/socket here, I understand it is permissible to use one in the case of very small trailers where a following vehicle can clearly see the direction indicators of the towing vehicle, though that still would not facilitate the connecting of a reverse light as obligatory lights must take precedence, the rear fog light being required as well as separate circuits for left and right tail lights so with a ground, stop lights and direction indicators = 7 pins.

it would be normal for the voltage reading to stop when the fuse is pulled as that is breaking the circuit
check the wiring and grounds in the trailer harness
there is a light control module but that would be the problem if the vehicle lights work with no trailer connection

it sounds that you have a voltage drop caused by bad wiring or a bad connection for the wiring of the trailer plug. you need to check the wiring of the trailer plug wiring back to were it connects to the truck wiring.pay attention to the condition of the plug connector itself.

Take a 12 volt test light... the kind that has a spring loaded clip to hold the probe into the wire, and insert it into the wire just before the trailer plug, vehicle side. (Blue I think) If there is a connection, the brake should see it and you should have an indicator light. When you activate the brake the test light should lite up. If not, move up to the brake actuator and test the blue wire. If there is nothing at the actuator, check the black wire. It is power to the controller. If no power there, check fuses again. If it does pick it up at the plug, connect the trailer and test the trailer itself. First ground to the trailer and then to the truck. Most problems are ground wire related. Then move to the axle and test it there. For the brakes to work, and even show on the indicator, there has to be a ground and power to the brakes themselves. There are 2 wires to each brake... a ground and a power. Test across both wires to make sure you have power and ground. You can also hook another trailer to your truck or another truck to your trailer. Lights can work through trailer ball ground. Brakes can't. leave the trailer unhooked from the trailer and hook the wiring up and test the lights. That usually will indicate a good or faulty ground. Also make sure there is a GOOD ground to the truck frame/body. Also try http://www.etrailer.com/faq-wiring.aspx. They have a great website with lots of info. Hope this helps. Mike

check the fuses under the hood including the ones marked as trailer fuses

Is your truck wired for a 7 pin flat plug? if so are you using an adapter? The adapters are made differently. One has the center pin as brakes and the other uses the center as Aux. power. Any chance of an animal getting to the wires? They are kown to chew on the wiring. Also try http://www.etrailer.com/faq-wiring.aspx. They have a great website with lots of info.

Do the truck lights work? If so, look under the back of the truck and check the connection to the plug. See if you are getting power there. Check your owners manual to be sure that the fuses are the "trailer stop/turn" fuses. The truck ones are seperate. Try http://www.etrailer.com/faq-wiring.aspx. Thes should help locate the problem.

try http://www.etrailer.com/faq-wiring.aspx. They have very good concise information that might help. You can take a 12 volt test light and check your fuses also. Touch each side of the fuse on the metal part that is exposed. Use your emergency flashers to do this. The turn signal/brake light fuse will only power when it is in use. Check owners manual as there are probably seperate fuses for the trailer and the vehicle. There is probably a connector under the vehicle near the back... usually above the spare tire that can come loose, but not likely unless it is broken. Hope this helps, Mike

I had this same symptom after I pulled a trailer that had light "issues". I tried wiggling the trailer light connector around until the trailer lights begin working. Next think I know they were not working again and now my left tail light was out. I thought it odd that is wasn't both lights out being on the same fuse, but checked wiring etc with no problems found. Mine turned out to be the "left tail light fuse" in the engine compartment fuse box. All of the parking and tail lights as independently fuse, You can tell which one it is by the guide on the fuse box lid. You will be looking for a 10 amp (red) fuse near the center of the fuse box.

rgant