What you've done seems to eliminate a bulb problem and could narrow it down to electrical. The bulb itself has 2 filaments. One for the lower voltage for the parking light, and a second filament for the higher voltage that runs the turn signal. There's a wire for each going into the headlight assembly. Examine the wiring and see if you can find evidence of the previous owner tampering with the electrical. See if any wires have been cut, crossed or are damaged.