Brake lights work when headlights turn on one blinks and goes out

Ok I've replaced the bulbs and still brake lights works just fine but when I turn the headlights on the passenger side taillights blink and goes out but still works when I apply the brake and turn signal I can disconnect the battery for like a minute or two and they come back on but do the same thing blink and goes out when u turn the headlights on I've tried everything as far as bulbs go I think it could be a relay or some type control module just don't know where to find them the owner's manual don't give no information on where anything like that is please help I work 3rd shift and have to drive at night