rear driver side blinker not blinking when headlights are on.
 

i just picked up a used 07 dakota quad cab ST 6 speed with 103k miles about 4 days ago. everything seems to be in good condition except for my driver side rear blinker when my headlights are on.
with headlights off it blinks as it should. with headlights on it immediately lights up as bright as possible (as if i were braking) and if i turn my blinker on, rather than blink it just stays lit up like i'm braking. this only happens on the drivers side rear blinker all other lights and blinkers work as they should. basically i'm just wondering if this is something small like a fuse/bulb or some faulty wiring, or if this sounds like a more serious issue. thanks in advance if anyone can help