Dear Glenn,
I have been leaving my keyboards in the trunk of my car for the last 6 years. Here are the problems I encounter and have encountered.
1. In cold weather it is impossible to read the LCD display until the keyboard warms up.
2. Cables are stiff in cold weather.
3. I had an internal smart media hard drive fail on me during the winter. It had no moving parts, and might have failed due to temperature fluctuations.
4. The external adapter for my ex-PSR740 failed on me. The cable got too brittle and broke.
You might want to lay out a new electric blanket on the bottom of your trunk. You can hook up a timer to start an hour before your performance and plug it in overnight. I am thinking of trying this.
Sometimes, I bring my keyboard in from the cold if my next day's gig isn't far enough away to warm up the keyboard.
Beakybird