Trailer Lights Work On One Vehicle But Not Another

Updated 12/24/2025 | Published 09/30/2014

Question:

I checked the trailer lights by hooking the trailer up to another vehicle and they worked but only the tail, turn and side markers work when I hook them to the VW.

asked by: Dan F

Helpful Expert Reply:

It sounds like the problem is either on the connector or on your VW.

First, you will need to have someone help you test your connector. Using a circuit tester like part # PTW2993, you will want to test each pin on the vehicle connector for function to make sure you are receiving a signal.

Check for any corrosion in the harness. Any corrosion can cause problems on a wiring circuit. Make sure the ground wire is securely fastened to a clean, bare metal surface.

If everything checks out on the VW wiring side, then you will want to check out the connector on the trailer side for corrosion or ground issues.

I have attached a couple of helpful articles you can check out.

expert reply by: Jeffrey L

Nick B.

6/9/2020

One of My work trailers lights don’t work behind 2 of my company trucks but do behind the other 2. The 2 trucks that this trailer doesn’t work behind have no problem with the other 4 trailers. Please help

Chris R.

6/12/2020

Now THAT is an odd issue. Are the two trucks that aren't working with the trailer newer (than the trucks that DO work with it)? A lot of newer models have a more sensitive trailer electrical system that can pick up more on wiring issues - something that older trucks may just power through. I might inspect the trailer side 7-Way for any debris or corrosion that could be built up inside the pins, cleaning out as needed. Also just make sure that the grounds are all still securely attached to a clean, bare metal surface. Let me know what you find and we can go from there.

Kyle

5/4/2021

@ChrisR Exact same problem as Nick has. The older trucks we have (2010,11,12) connect with one trailer fine and lights and brakes work. However, all 3 struggle and mostly cannot connect with our second identical trailer. The newest trucks (2018,19,20) and rentals we occasionally drive connect fine with both.

Jon G.

5/5/2021

@Kyle I would follow Chris's advice for your situation as well. Wiring can be a bit finicky at times and so when you start seeing problems like this it's best to inspect all of your wiring to see if you can find anything that is damaged, pinched, or corroded and get it taken care of.