2006 Mariner tail light problem
 

I have a 2006 Mariner that has a weird left taillight problem. When the lights are on, the large filament (left side) glows as the tailight, rendering the turn signal inoperable. L turn sig works fine with lights off.When lights are on, L turn sig flashes twice as fast as normal. Also, (lights off) when stepping on rhe brake, the left front park light comes on. Tried new bulb-no fix. Cleaned and tightened ground wires to chassis-no help. Small filament seems to get no voltage, though socket shows short when connected to chassis wiring. According to the wiring diagram, everything else run by the park light circuit works fine. Any help on this would be greatly appreciated!:confused: