Could be...

As I turned on the heat/air it quite quickly (inside 2 minutes) jumped from 24C to 29C.
After about 5 minutes it jumped to 31C. At this point I was entering and exiting the Hijack menus, checking the temperature about once every minute.

Maybe I should bring the cig.lighter adaptor next time, so I can hold it out the window while driving - that should cool it drastically as it was -9C here as I drove to work.

/Michael