I NEVER leave my kb in the car overnight, but there are times when I might have to keep it in the car during the day for a gig later that night... Winter or summer, temperatures can be somewhat extreme ...
I use a soft, fluffy towel around the kb to help protect it from condensation ... When I get to the gig, I unload and unpack the kb first, so that it gets aclimated to the temp of the room while I am setting up the rest of the gear, then I turn it on for a while before playing ...
These babies can be SO tempermental!!!
t.