Strange brakelight problem

[gkesseru]

I'm pretty good at electrics, but this one's got me.
Just put the truck harnesses in....
All lights work except left brake light.
Left turn signal works, front and back.
Four ways work all around.
Putting right turn signal on and brakes at same time is only time left brake light comes on.
Battery cable goes to engine block, battery is also grounded to the body at the radiator support, body is grounded to the frame, back lights are grounded to the frame.
?????????????????