Yeah, I know salt. Nobody uses more salt than us here in Canada. In any case, I hate to be the bearer of bad news but doing what you are doing can greatly accelerate rust. Sorry to hear about this but I am pretty certain it is contributing to your problems. I speak from experience on this one. Not with my 4Runner but with a previous car I had while I was renting years ago at an apartment building that had underground heated parking. It will really cause problems. I would suggest that you only heat your garage when you need to work out there or something, otherwise leaving it unheated is definitely the thing to do...Good luck...It is the abrupt change in temperature that is killing you man...
Cheers