Have you tried to do a follow-up with the companies that haven't responded back? In my experience, sometimes portfolios (esp. hardcopies) can get lost or placed into the wrong hands in larger studios. If you don't do a follow-up, the studio might not even realize that they have yet to go through your application.
Usually, I will ask the instructor or dept. chair (anyone who knows someone at the company) to do a follow-up for me. It really does help if you know someone who may know someone that actually works in the studio you are applying to. I've had similar experiences before where I wouldn't even get any acknowledgment of my existence from some studios. But after I asked my instructor(s) to make some calls, the studio kept a regular correspondence with me and would even answer little questions I had.
Hope that helps. I think it's important for graduated students to keep an open communication with the career services dept. at their school or to keep in touch with their instructors.
Also, you should definitely apply to more than just car design jobs and keep updating your portfolio as needed.
CCS Automotive Senior
"Use no way as way, Have no limitation as limitation" - Bruce Lee
"Take notes on the world, there will be a test" - Chris Bangle
sk8nrail.deviantart.com - online teaser portfolio