71 F100.
Installed a new switch and new bulbs. The left brake light works sometimes and not sometimes. Seems like when I have time to fool with it, it works. I've even tried wiggling wires from the switch to the light to make it go out. Not happening. Signal and park lights work just fine. Even pulled the light assembly out, checked grounds and connections. Everything looks good. Anybody have a suggestion to figure this out.
If the turn signals work fine, then most of the wiring and the bulbs and sockets are OK because the signals and brake lights use the same filament in the 1157 bulbs.
So, as far as I know, the possibilities are:
Brake light switch - But that doesn't explain why the left brake light works intermittently.
Emergency Flasher switch - Same comment as above
Turn Signal switch - Ah . . . That could be the problem. Maybe it's not letting the brake light voltage pass when the turn signal switch is in the "off" position? If the TS switch is bad, that could explain why the left brake light works sometimes and the right one never works.
Try moving the TS switch lever a little up or down while you apply the brakes to see if the brake lights will come on. This won't be a Go-No Go test but, if the brake lights are affected by moving the TS switch lever slightly, I gotta think the switch is bad.
Good luck.
Bill
68 F100 Ranger
73 F100 Ranger XLT
Believe me, my young friend, there is nothing - absolutely nothing - half so much worth doing as simply messing about in garages. (Apologies to Kenneth Grahame.)
68RGR,
That was an excellent solution. Moving the TS lever proved to be the key. When it went out again I moved the lever and the light came on. Moved it back and it went out. Now all I have to do is figure out how to change the TS switch.
Anybody know of a good tutorial or how-to videos?
Thanks again.
Glad that narrowed it down for you. You might want to post the turn signal "How to" question in a new post.
Bill
68 F100 Ranger
73 F100 Ranger XLT
Believe me, my young friend, there is nothing - absolutely nothing - half so much worth doing as simply messing about in garages. (Apologies to Kenneth Grahame.)
Got the TS installed Friday. I decide it couldn't be that complicated.
Seemed like the hardest part was getting the steering wheel puller hooked up.
But now everything works as it should. All the time.
Thanks again 68RGR.
I've even tried wiggling wires from the switch to the light to make it go out. Not happening. 
Signal and park lights work just fine. Even pulled the light assembly out, checked grounds and connections. Everything looks good.
Anybody have a suggestion to figure this out.
Moving the TS lever proved to be the key. When it went out again I moved the lever and the light came on. Moved it back and it went out. Now all I have to do is figure out how to change the TS switch. 