Hey newguy...
You gotta leave at least two of your car windows down 1cm to let the heat escape, as otherwise it can hit 60+ degrees in there and potentially delaminate your board (and for certain melt your wax...).
I used to do this when I was still on boards small enough that I could put them in the back of the Torana! In those days we were still using the board 'socks' type covers... I used to put the board in the car wax side down and chuck a couple of beach towels over the top - 1) to 'hide' the board and 2) to provide an extra layer of insulation...
These days I'd be more worried about theft than damage to the board...