I have a problem that has me stumped. Here is the quick and short of what happened. I hooked up boat trailer and lights and everything was working as it should. Then, somehow the tunning light wire and the ground wire on the Trailer crossed ultimately melting a lot of the trailer wiring and ruining my trailer light adapter (7 to 4 plug). I bought a new adapter, rewired the boat trailer wiring but cannot, for the life of me, get the trailer tail/running lights to work at the truck's towing adapter. Here is what I've tried thus far:

Checked all fuses both in the cabin and under the hood.

Checked the Towing relay, it's good.

Using a test light I have confirmed that all the other terminals at the truck adpater are working, the running light connection is not getting power. I even unhooked the factory adapter and tested the terminals that connect to the adapter.

I bought another relay towing adapter thinking mine somehow took a crap. Put the new one in and same issue.

I checked the terminals and connection at the adapter itself, nothing melted or abnormal looking. I unhooked the adapter and tested the "ground" to "running light" connection - no power (using test light)

From the wires I can see doing all this none look melted or abnormal.

The tail lights on the truck are fine.

Please help, is there a mysterious fuse I am not seeing, relay or some other way I can find the problem??

Never realized how much I towed at night until this has happened! Thanks for the help folks!!

Bueller??

Sounds like a ground issue, here is the Toyota wiring schematic. Hope it helps and good luck.

Attached Images

060trailertowing.pdf (51.5 KB, 607 views)

thanks Morphious, I'll give it a shot. Where did you get this diagram? I have the Toyota repair manual but haven't seen a wiring diagram like this in it

Well, long story short, double, triple, and quadruple check ALL fuses. After using the wiring diagram morphious provided I was able to conclude there weren't any additional converters, relays between the connector in the back and the engine compartment. I even bought a new "relay towing converter" thinking my problem was there. Not only did that not work, but the trailer/running lights don't even go through relay. I used my voltmeter to see if I was getting power to the towing relay by engine, no-power. Only place left was the main fuse panel. Low and behold, I found a 40 AMP square fuse that was blown. Two months of pain and agony were caused by a lowly fuse. Thanks again for the help Morph

I am glad you were able to figure it out, here is a link to the diagrams that are stickied to the top of this forum. Very useful information.

Service Manual Download Links.