I'm with the no problem camp on this one too. I've had instruments in my trunk on the road through all kinds of weather. Through Arizona in the summer. 112*f, (which translates to much hotter in the trunk) to winter through Colorado. Never a problem. Think of all bands that tour. Their stuff is in a hot/cold non insulated truck or van traveling through dry hot to humid, to damp and cold. From extremely hot or cold into well air conditioned or heated venues.
