Hey Scotty, when my heater is off I have heat blowing through the defrost when I'm driving down highway , and not as much when in driving in city. I changed the blower mower resistor , but it's still doing it. I have 2009 police interceptor. What do u think it could be?

it's going to default position, often a failure in the control unit does that. Have a pro scan it, not something you can figure out without a computer scanner capable of testing HVAC systems