Well just a regular usb wired gps puck like the bu-353 can be placed in the dash behind metal and plastic, and still receive a strong signal.
I just recently (4 days ago) bought a BU-353 and I shoved it in my dash behind metal and plastic. I got a ton of satellites and a quick lock from it. Even in my garage with the door closed I can still pick up a good signal.
So an external antenna really isnt neccessary.
If you are really wanting to use an external antenna, then you could get a oem chip (like from sparkfun) and connect it that way. They even have a 5hz chip to buy.