I got the 1999 f-150 with towing package.
I got only power in the left brake/signal trailer light. Nothing else.
I verify the power at the connector and there's no power (only in the left brake light).

I disconnect the main plug at the back side of the harness connector and measured the voltage when lights on (o vdc).
I measured when the brakes were pressed, and only 12vdc at the pin that goes to the left brake lights.

1. Is there any fuses that serves this power?
2. I verify a little fuse (10amp) at the fuse box inside the cab and is ok.
3. Anything I must verify?

I don't want to start taking all the cables apart and re-cabling the lights in the old style way. I want the factory towing package to be original

 

I have a 1998 that had a similar problem. Inside the engine compartment, there are two fuse boxes. The mini one has the trailer towing light fuse in it. On my 98 this box is the small one on the drivers side, towards the rear of the engine compartment. If I remember right, there were only two fuses in it. Mine was a 20A.

 

very good!!!

Mine has a big black box, and the fuse was a 20 amp (like you said). I verify the trailer lights (for shorts) and found one of the connectors shorted to ground. I fix it, replace the fuse and KABOOONN!!!! eveything is working again...

Thanks Man, and KEEP ON TRUCKING !!!