Brake light problem

I'm hoping someone can give me a head start to troubleshoot this:
'66 Mustang Coupe with left brake light not working. What has me confused is everything else works and its a single bulb (per side) system. Both taillights work, both turn signals work, right brake light works. Any ideas to save me some time?  

Check to be sure tail lamp housing is clean, secure and properly grounded and the electrical pig tail that connects to the back of it is clean and works. Sometimes just rotating the pig tail back and forth will make it work again. They can go bad. If you have a tester check the insides of the connector with the brake pedal depressed to see if you have current to the tail lamp housing. If not, then the pigtail could be bad or you could have a broken brake light wire in the trunk. Just keep testing along the brake light wire until you get current. If you go all the way back to under the dash without current then you probably have a bad under dash connection or t/s switch.
One way to check if the left tailight housing is OK is to move the pigtail connector and bulb from the right tail light (that works) and temporarily put it on the left. The left brake light should then light up. If not, then you know it is in the left brake light wire/pigtail or under dash.  

I beleive that is is just burnt out. Mine are duel-element bulbs and one element could be burnt out.(the brake light side) and the dimmer of the two (the tail light) could still be good.  

Scw78,
If the tail-lights and turn signals work as you've stated, its a failure in the turn signal switch (brake light circuit within the switch). To confirm, turn the left signal on and step on the brake, the light should not continue blink(it should go dark).  This was a common failure mode in Ford TS switches.jlg