Hi, I'm in PEI, Canada, so I get this a lot.
I did it this week this way:
1. Boiled water just to the boiling point, not rolling.
2. Put a litre into an empty jug, like a laundry detergent bottle.
3. Add one half-scoop of Oxiclean. If this is a concern to you, use carpet shampoo. I don't think it matters. Don't put a lid on (Oxiclean+shaken closed container+hot water=boom), but mix it.
4. Poor it on the areas that are salt-encrusted.
5. Agitate with an carpet/upholstery brush.
6. Extract. A 5hp wet/dry vac will do okay, but dry the car before you venture outside again.
There are other threads on the "Oxi Method," and people against it, for fair enough reason. It is a mild bleaching agent, but companies like Zep do sell an automotive version. I think any carpet shampoo will do, I use Automagic.