Towing Issues: Ford Explorer cargo trailer interior lights not working, digital volt meter, fuse box

Question
QUESTION: I hooked up a brand new cargo trailer to our explorer and interior lights only not working, all exterior ok.  all fuses in explorer appear to be in place, non burned out and none missing. these work as seen at dealer... Any suggestions?

ANSWER: Is the problem with the trailer or tow vehicle?

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QUESTION: No problem with trailer, interior lights worked at dealer when hooked up to their truck.  Think its our explorer with tow package, that ford didn't hook up 12v lead somehow - everything works except interior lights on trailer

Answer
Look in the fuse box under the hood for aux. feed or trailer feed (if there is one). I am assuming you have a 7 blade rv type plug. There is a 12v feed for aux. hookups but many times a fuse or relay may need insertion as the factory will leave them blank. Here is a link http://www.etrailer.com/faq_wiring.aspx for wiring that may help for a quick test. I suggest a digital volt meter a test the truck plugs terminals for confirmation of voltages before you get to deep.