trailer hitch wiring harness troubleshooting

[YoTerryh]

I've purchased a cover and cargo net to protect my mountain bikes while transporting them on my rear rack. Then I realized my tail lights would be hidden, so I bought a kit to add lights, and next discovered the pre-existing trailer brake-light extension plug is not working.

I've never pulled a trailer or tried plugging in auxiliary tail lights on my 2001 E250 before, so I just discovered this now. I'm not confident that resolving this is within my skillset, but willing to try. How do I proceed to diagnose and repair this issue?

 

[carringb]

Factory trailer wiring? That is fused separately in the Power Distribution Box under the hood. It’s the 3 mini fuses in the corner. They’re only 10 or 15 amps, so any bad wiring on a trailer will trip those.

 

[mothgils]

Factory trailer wiring? That is fused separately in the Power Distribution Box under the hood. It’s the 3 mini fuses in the corner. They’re only 10 or 15 amps, so any bad wiring on a trailer will trip those.

Slightly off the OP's post but do you know what the part number is for the factory 7-way wiring harness that runs down the frame rail?