bhai log,
It's not so much the air. In fact Karachi is drier than Lahore most of the times in the year. It's one side sea and three sides desert.
Rusting takes place under moist and humid conditions. Karachi does go through hot humid spells but not much rain. Lahore and most areas in Punjab get copious rainfall throughout the year.
The main rusting agent in Karachi for cars, is the tube-well supplied water in the service stations across the city. That water is a lot more harder and saltier than normal tap water, that which is available in abundance throughout Punjab.
Just my 2 cents.