Hey mate this is a common problem and its actually happening to me at the moment.
It is an expensive fix and would be more worth your while to do what im doing and upgrading to a double-din headunit (Clarion CX501 to be exact)
The auto a/c is a feature of the car that controls the fan speed etc depending on the temperature and the engine temperature so if you want to warm up the fan speed will be very low until the car is warm and the heater will actually work.
Anyway the symptoms are the same at mine.
Chris rogers covers it here as well: http://www.skylinesaustralia.com/forums/topic/240794-v35-faq-read-first-before-posting/
Heres an example of the easiest way for you to install a double din: http://www.ebay.com.au/itm/METRA-99-7604-INFINITI-G35-03-04-SILVER-DASH-KIT-COMBO-/181072427505?pt=US_Car_Audio_Video_Dashboard_Installation_Kits&hash=item2a28c1f9f1
Although the metra kits arent perfect and it would be better if you could source a JDM double din fascia (found on base models, same as M35 stagea)
If you are worried about the 18deg, im pretty sure unplugging your battery will turn the aircon off.
good luck!