Left brake light/tail light issues

[ace]

Need some help here. I have a 1969 standard Camaro with a left brake light/taillight illumination problem.
Scenario one:
  Left rear brake light when depressed is dim.  The bulb is good (checked for continuity). A visual inspection of the light reveals that the "brake" filament does not light up, but instead the taillight filament is lighting up. . Thus I have a dimly lit bulb.     Voltage at the socket to the unlit filament (with brake applied) is 10.85.  Voltage at the socket to the "lit" taillite filament is .36. (Again, why there is any voltage at all to taillight filament is not understood).     

  The right side  brake light lights up correctly (that is, brake light filament is what is energized).   Voltage to the right side socket is exactly the same as the voltage to the left side.

Scenario two:  Brake pedal depressed and headlights on
Now, when the headlamp switch is engaged with the brakes depressed, left brake/taillight goes completely off. Neither filament is illuminated        The right side taillight and brake light work fine and shows excellent lumination. Voltage to left socket is 10.12 (for taillight filament) and 10.28 for brake filament).    I have voltage but NO ILLUMINATION.   Right side socket voltage:  10.2 for light filament and 10.2 for brake filament .   GOOD ILLUMINATION

Scenario Three:    Headlights on - brake NOT depressed.   Both taillights are illuminated;   Voltage left side socket  10.12 for light filament and 1.26 for brake filament (why is there voltage to the brake filament???);  Right side 10.08 for light filament and again, 0.9 for brake filament.

The left side socket is fairly new.    Tested socket for continuity to ground.  Socket is good from what I can tell.

ANY THOUGHTS ON WHY MY LEFT SIDE SOCKET IS USING THE TAILLIGHT FILAMENT AND NOT BRAKE FILAMENT FOR ILLUMINATION WHEN THE BRAKES ARE DEPRESSED ?  2)ANY THOUGHTS ON WHY LEFT TAIL/LIGHT FILAMENT AND BRAKE FILAMENT DO NOT ENERGIZE WHEN HEADLIGHTS ARE TURNED ON WITH BRAKES DEPRESSED?
 
          NOTE:  TURN SIGNALS WORK BOTH SIDES in all three scenarios

Any thoughts and guidance is appreciated.

Ace

[Kelley W King]

You have a cable running from the dash to the trunk through the car. If it gets pinched or it is bad weird stuff can happen. Locate the cable under the dash drivers side and start checking it for shorts or grounds. Getting some minor voltage at the sockets on the wire that shouldn’t,t is normal with a sensitive meter.

[ace]

Thank you Kelly for the response.  Will follow the harness back to the dash and try to find the problem.  Will report back what I find.

Ace

[olddragracer]

You have a bad ground at either the tail light socket or between the bulb base and the socket. Both filaments in the bulb have a common ground in the bulb base. If you have a bad ground the power flows from one filament to ground in the bulb base then from the common ground through the other filament and then to ground in the opposite circuit. The filament with most resistance will light up. If you have both the brake light and tail lights on the there is power to both filaments so neither filament will light as the is no ground after the bulb base. [or socket]

[stevieBB54]

Subscribing here, good info. I am hearing these first gens are notorious for bad ground issues, especially after a repaint. Not to hijack thread, if I may add another question on this same topic. I have similiar issue on a 68. Brake and tail lights work, but one brake light on left side lights up when applied, then goes out while the other brake lights stay lit (RS)
To OP, check your bulb bases, old 1156 and 1157 bulb contacts flatten out and can touch each other or short to bulb or socket housing. Happened to me.