Street ka heater system question

Hi all

Anyone answer this?

Went out in the streetka today  with the roof down 

noticed that the air vents were blowing hot air (phew)

 control knob was set to cold (lowest setting) no amount of fiddling with the setting made any difference still hot air only(also air con switch not lighting up)

when i got home put the roof back up ran the engine and tried the air con again  switch worked as normal - after a few minutes cool air again!

so the question is - is the cars heater system set to a default to supply hot air whenever the roof is down?

if so i am guessing theres a switch or sensor of some kind to send the system that info,if so where is it and can it be disabled without effecting anything else, i would quite like cool or if not ambient air from the vents with the roof lowered!

thanks in advance for anyone who takes the trouble to reply

regards

Steve