1992 civic dx hatchback. The brake lights work, but the 3rd light is bright and the other two are really dim, as well as the fact that the driver's side rear directional does not work. When I turn on the headlights, the 3rd brake light goes on and the other brake lights don't work at all. Anyone out there have some advice as far as where to look? I've tried everything I can think of and I'm getting nowhere fast.

 

The bulbs on the sides have lost their connection to ground. Either the sockets are corroded or the ground wiring is bad. This causes the brake and tail light circuits to interact like you described.

On hatchbacks it is common for the wires to break where they go between the hatch and the main body of the car, since they flex there every time the hatch is opened. The outer appearance of the wire may be OK but the copper inside breaks.

 

O.K., that makes sense, but how do I determine which is the ground wire? I've tested all of them with my multimeter and all of them go to ground. Is there anywhere i can get some kind of pinout or schematic or something on the color coding? It's not like I'm a tyro on wiring, I helped my father build a TV when I was six years old, I've since rewired a whole bunch of guitars and built from scratch, breadboarding, a few stomp boxes. I'm not ignorant, but I think maybe my next step is ripping up the carpet on the driver's side and looking at the harness there. Also, this particular car does not have the brake lights in the hatch.

 

I've also tried to set up an independent ground on on both sides and also swapped the rear light harnesses which didn't change anything

 

Ground wires are almost always black. I think this is the case here. You can also test voltage from the shell of the bulb to a good ground, with the light on, the voltage should be near zero if the bulb is grounded.