Yank all the bulbs in the vicinity of the left rear side. I think you have a filament shorting or a bad ground going through another filament. Try again with empty sockets.
What are these clearance lights, are they low and facing sideways so perpindicular traffic can see them, or are they the ones up top that say you are a tall RV? The former should sometimes blink in "opposite unison" with the main blinkers; the latter should never blink.
The best place for problems are bulbs that do two duties, park lights and blink. Could be in front or in back.
The cruise is an interesting twist, it will have a brake light sense circuit. Might have to check your diagrams to see if it's at the pedal or if it's a seperate switch near the master cylinder that detects fluid pressure... fords are funny like this.