Brake Light Problem

Developed some issues with my driver side brake light after the bulb burnt out. During the initial changing of just the bulb noticed the socket for the brake bulb was recessed, and twisting the new bulb into place was difficult and caused the whole socket to spin. Eventually the inop brake like problem came back, I tried seating the bulb more securely, the brake light bulb just wouldn't illuminate and if I moved the bulb just right in the socket all the brake lights would come on without the brake pedal even depressed (only did this with headlights on/brake light inop other than that). I just recently ordered a brand new light assembly and replaced the entire brake light assembly. Everything is working better but I've noticed two issues. It appears only one filament is illuminating with the brake pedal depressed (headlights off). And when I turn my headlights on the driver side brake light illuminates without the brake pedal depressed. Passenger side has no issues. Turn signal, back up light work fine on driver side. My assumption is a possible diode in the lighting circuit has possibly gone bad, but without the proper schematic it makes troubleshooting difficult. Any ideas or suggestions?