The other day I discovered an issue with one of the two lights in the drivers side taillight of my ‘74. Judging from how the passenger taillight functions, both individual lights perform the same actions: come on with headlights, blink with signal, glow brighter with brake. One of my driver side lights works as described. The other one comes on with headlights, but goes out with both signal and brake. The bulb is an 1157 double filament. During the malfunction the first filament glows with headlights but then goes out with signal/brake instead of the second/brighter filament glowing. Before I dive into the service manual my question to anyone who can help is, am I looking for a bad ground or something else? My electrical capabilities are pretty basic. Thanks!