When was the last time everything worked correctly? What has been done since then?
These classic Mustangs used a 2 filament bulb to do 3 things. So the wiring is not easy to understand unless you really work with it. I had to draw my own simple version to get it, which I just scanned and attached.
Sounds like you have a chaffed wire problem or a bad turn signal switch. The stop and turn wires combine at the turn signal switch
1968, hardtop, 289, C4, seafoam green.
1966, hardtop, 289, original 3 speed, on the road and fun to drive.
1968, hardtop, 200, 3 speed, new project car, Nov 2015
Check bulb- it has two filaments: One turn one stop.
Check ground on left brake light.
This can be done by using a jumper wire from ground of bumper or other ground and other end to Brass Receptacle of light- which is ground, then apply brakes and have some one check to see it lights are on. Bad grounds can cause strange results.
Hello. It is true that the tail lights have a two-filament bulb, but, the turn signal and the brake lights use the same filament. If either the turn signal or the brake light works, the problem is not the bulb, or any wiring issue in the trunk area. What you have is almost certainly a bad turn signal switch. If the problem was upstream from the turn signal switch, neither brake light would work correctly, because there is only one wire going from the brake light switch to the turn signal switch. If the problem was downstream from the turn signal switch, both the brake light and the turn signal would have the same problem on the same side. Since both turn signals work correctly, that means that the problem isn't down stream from the turn signal switch. That leaves the turn signal switch.