Alright, so I pulled my dash to replace some bulbs in my gauges, and to take out all of the wiring that the P.O. had put in for his amp install (which is no longer there), as well as to remove all of my aftermarket air conditioning wiring. When I went to disconnect the wires that go to the brake light switch (I'm not sure if this is the correct name, but I believe it works) one of the wires ripped off of the unit itself (I know, I probably pulled too hard). I pulled the unit out, and soldered it back onto the connector, then tested it with a multimeter, and it was working just fine. I put it back onto the car, and the drivers side brake light now does not work all of the time. I had messed with it for a few minutes to see what the issue was, I took the bulb out, cleaned it and the connector with electrical cleaner spray, and it would work half of the time. I finally got it to work, but I was informed today that the light was not working on the driver's side.

I don't believe it to be the brake light switch, because it's just an intermediate, correct? If so, that means that both brake lights would not work if it were defective. This leads me into thinking that there is another wire that is disconnected/loose/my connector for the light itself is bad (upon visual inspection, there was no corrosion). I'm trying to read this wiring diagram, and I can't understand it for the life of me. What color wiring should I need to find/follow, and where are the wiring connectors located at? Could I have a loose connection going to the drivers side light that only works some of the time, or could it be another issue? I'm really at a loss for words on this one.

 

The bl switch is on or off for both lights. Intermitant suggests a bad ground, somewhere. If the bl doesn't work try the turn sig, it uses the same bulb. A likely place for the short is in the rear loom, somewhere.

 

Theramz is right, it has to be "downstream" (to the left) of the passenger side light. They not only use the same switch, they share the same hot wire (green with red stripe in my '78), and they share the same multiple-grounded cable (black). The rear side marker lights and power antenna also use that ground. The closest to-earth ground is in the dead center, back. In the interior, on the opposite side of where your license plate mounts. If your antenna and your left-rear marker light work okay, then it's not the ground. In that case, it's either a short or an open of the hot wire somewhere between the working passenger-side stop lamp and the intermittently failing driver-side stop lamp. The connector / plug is the most likely spot.
The easiest way to test this is to first remove your interior trim panel (sorry, I know that's a pain in the butt). While someone holds down the brake pedal, making the brake light(s) come on, move the harness around, and see if you can make the driver side go on or off. This can help show where the trouble is, either a short (the green with red has been abraded and is grounding on the body), or an open (typically a section of wire you can bend around, making an internal gap open and close, like when an extension or lamp cord goes bad). Thankfully, it's a relatively easy area to access, and not a long length of wiring to test, being at the far end of the interior harness.
Good luck,
Dave

 

Another thing, since you pulled the dash you may have damaged a terminal plug. The brake lights have their own fuse, so check the terminals on the fuse box pigtail too.

 

bulb ended up burning up, and was working intermittently