93 vtir rear demister

hey guys just a question about my rear demister. when i push the button to turn on the rear demister it doesnt always work. It is as if the car knows when it is cold enough for the demister to work, and when it does work i can turn it on and off as i please. is this a feature of the car? im pretty sure the demister button is the problem but then again i could be wrong. any1 else have this issue? and my car is also a imported model from nz, so i dun have the cluster light adjuster thing as well. so I dunno weather my car being imported should make a huge diff to the specs.

cheers