I have a 07 350z 5AT. My heat and ac works fine, the problem is that there's no in between. It's either cold or really hot, if I keep the thermostat settings to cold it's cold but if I click it two clicks up from the lowest setting...blazing hot. What could be the issue? I've replace a new thermostat, still nothing. Thanks

 

Stock radio or aftermarket?

 

Aftermarket, it just start doing this a couple of months ago. I've had the same radio for a couple of years.