I don't know about car trunks but I kept a 1911 in condition 3 (thats full mag nothing in the pipe, right?) under the seat of the truck for about 5 months and it never showed any signs of rust or anything. I took it out about once every month, looked it over, and shoved it back under the seat. It might've helped that I had it zipped up in a handgun case. It wasn't anything weather proof though, just $10 zipper canvas deal from Academy.
I live in south Mississippi, btw. The most humid place you can be in the US.
I live in south Mississippi, btw. The most humid place you can be in the US.