alright here is what the ole 4runner is doing:

lft turn signal
front and back lft are normal
RT brake light flashes too

brakes
left turn signal solid on front and black
no left brake light

tail lights are normal while head lights are on (other than the flashing brake light with lft turn signal)

I checked my trailer wiring harness and all seems fine. What should I check next?

 

You most likely have a bad ground in one of your sockets or a bad bulb that is trying to ground via the brake system.

I would look at the left side first

 

On the passanger side rear,look at the harness on the frame where it passes over the rear axel housing.See its broken or rubbing the frame.

 

The harness on the frame looks good I will check the bulbs.