Trying to figure out why interior lights on TrailsWest 2-horse trailer aren’t working, what am I missing?  Are there trailer fuses, or a circuit panel, and if so where would they be?  Internal battery reads 11.9V, but I’m not sure if that only powers the emergency brakes.

The 7-pin connector from the Chevy truck is providing power on all the pins; the 12-v pin and Aux pin are powered when the truck is on, and the exterior lights work fine when the truck lights are on.  Junction box under the trailer tongue is crammed with wires/connectors, but it doesn’t seem to be a very logical place for fuses.  Any advice will be appreciated.  Even a wiring diagram would help.

If the truck is supplying 12 Volts to the Aux pin, then obviously the problem is from there to the interior lights. Have you checked to see if you have voltage at the light? That would indicate no ground, if there is no voltage at the light then you have a wiring problem from the plug to the interior of the trailer.

Of course always check the bulb with a known good one............

Trails West says the problem is most likely bad connection or corrosion within the trailer junction box.  Trailer is 2003 model and box cover appears too loose so I tend to think they're right.  Should know by the end of the day.

They provided their standard wiring for 7-pin connector indicating:  Black - Hot Wire - Dome and Load lights.  Their other pin / wire colors are in attached pdf diagram.

Trails West rep also said that the Chevy truck connector 'hot pin' usually isn't wired to provide power to trailer when truck is not running - unless the dealer makes that change when the truck is originally delivered.  That's not what is causing my difficulties, but it does need to be changed to have interior trails lights when truck is parked (just need to be sure they are all turned off when not needed to avoid running down truck battery.)

Quote "Trails West rep also said that the Chevy truck connector 'hot pin' usually isn't wired to provide power to trailer when truck is not running - unless the dealer makes that change when the truck is originally delivered." 

I believe the above statement to be false..... I think your Trails West guy is thinking of Ford. The Ford truck has to be either running or at least have the ignition switch on.

What year model is your truck?

All of my GM truck's hot pin in the 7 prong plug is hot all of the time, doesn't matter if the truck is running or not, just so long as the 40 amp fuse is in place under the hood. If the fuse is not installed the connector will not be hot at any time, running or not.

Here's a chart that may help you....  http://www.horsetrailerworld.com/home/wiring.asp

With apologies to you and Trails West both; thanks for making me look back at the fuse box.  The 40-amp fuse is in place but it is connected to an external stud on the side of fuse compartment, which doesn't have a nut to complete the connection!  I had read the diagram attached thinking the left side was the truck socket noted as "car end"; when the hot stud is at the 2 o'clock position, and I really wasn't getting the 12v from the truck even though I thought I was. 

This is a 2009 Chevy which daughter has had for six months and has just noticed she didn't have the interior lights in the trailer!  Thanks to everyone.