TR4/4A left brake lamp not lighting TR4A

[canyon77]

Getting 12 volts to contact; left taillight lights up but left side brake light doesn't; bulb is good (repl with new bulb and verified good bulb)

 

[Basil]

Not sure what car your have but you need to post these kinds of questions in the appropriate Marque forum (for example, if you have an Triumph, post in the Triumph forum, etc). You posted in the "Forum Navigation Help" section, which is reserved for asking questions about how to navigate around in the forum. I am going to "move" your post to the Triumph forum but leave a "redirect" in the Forum Nav Hel section for a few days.

Basil

 

[KVH]

Getting 12 volts to contact; left taillight lights up but left side brake light doesn't; bulb is good (repl with new bulb and verified good bulb)

My guess is it's right there in your wiring, especially if the other brake light works. Replacing bulbs isn't always a "tell all" for me. I've put in new "bad" bulbs, and I've had bulbs not work properly (like an intermittent blink or fast blink) if a bulb on the same side has a problem. I'd look first real hard at the bulb and then the wiring, and be sure all other bulbs work, and that the other brake light goes on.

 

[Geo Hahn]

Could be a poor connection including a poor ground where the base of the bulb contacts the socket. The tail light is the dimmer filament and draws less current so perhaps it is able to illuminate despite a poor ground.

You say 12V at the contact but I usually test that with a test lamp rather than just a voltmeter as looking at just voltage can be misleading.

 

[TR3driver]

The tail light is the dimmer filament and draws less current so perhaps it is able to illuminate despite a poor ground.

If you try the brakes with the tail lights on, does the lamp go out entirely? That's a classic symptom of a bad ground, because the brake light filaments form a "sneak" ground path for the tail light filament; but only when the brake lights aren't on.