Hot water with some distilled white vinegar. Seems like the acids in the vinegar help dissolve the salt better. Note that "salt stain removers" for shoes are basically just vinegar. Rewash/rinse well unless you like the smell of vinegar
Just to add to what Accumlator said I think the ratio is 1:1, water to vinegar, for dissolving the salt. I got sick of cleaning that up at the end of every winter and got the factory rubber mats for my car for winter use. It is much easier to clean. :xyxthumbs
I was about to ask "how the heck did you get salt stains in the car?" (i'm thinking profuse sweating?) lol... Being from the south ya know lol... man i felt dumb getting to the bottom of the post lol.
I've always been able to get the majority of it out when it was dry using a good vacuum w/crevice attachment, followed by a hot water/woolite extraction to get the remainder.