Wiring problem..

[kitchene]

I'm rewiring the rear brake/signal/backup lights and the trailer connection, 7 pin flat. I've got pretty much everything sorted but I can't find the auxilary left turn signal for the trailer. I think it's supposed to be the yellow wire but it's giving me no readings. I've even traced it back to the plug in the engine compartment and I get nothing at the plug into the cab. Is this the left signal or am I barking up the wrong tree? it's a 2000 f550 cab/chassis if that matter. My dads 02 f250 is nearly the same and the yellow is the left signal on his. only difference is the ground is black on my 00 instead of white like on the 02

[kitchene]

was a fuse. didn't know they had fuses for the blinkers for the trailer.