As a HAM/SWL you may eventually come to the point when you ask "Why are there no Shortwave Radios in Cars?", aside from the small market for such specialized equipment you may discover that the best radio is the one you already have. Rather than messing around with a crummy in dash Shortwave radio, of which you probably only use three or four stations get a short range FM transmitter and build a dock for your portable shortwave unit.
Understandably you are going to have to do your own math and work to get a proper dock, antenna, and FM transmitter for your vehicle. It can be easier than trying to rewire the proprietary connectors in your vehicle, and much more affordable if done right. On the odd occasion I choose to listen while on the road I use a FM Transmitter from DealExtreme - a respectable vendor from Hong Kong. One antena configuration was in a triangle wire antenna between the front seats to the two clothes hooks in the back - it was too complicated and unweildy. A modified active loop antenna strapped to the top of the radio solved the problem.
Find the radio for you and take it with you.