Leave the AC off in your car or truck. Keep your kit at around the same ambient temperature as the outside. Fog generally occurs when you move from cooler to warmer in high humidity. Living in Florida, we have this issue quite often.
On my truck, I leave the windows somewhat open when it's raining. It rains a lot here. If I get rain inside, I'll close that window. I also sometimes use a locking tool box in the bed, especially if I don't have to leave the truck for long. I bought the toolbox for camera gear storage, not tools. Sometimes it's needed when three or four of us go into the field and there's no room for gear in the back seat. As an avid fisherman, it serves well there too.
Even if the lens fogs, it generally clears up in short order, at least for me.
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Cheers, Craig
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