One thing you may want to consider is a Compact Flash based device. These can plug directly into some PDAs and, using a simple (cheap) adaptor into a PCMCIA slot of a laptop. This would work well with a PDA mounted on the dashboard but perhaps it would not be so good in a laptop inside a car. Mind you, I've found my 'wristwatch' GPS works reasonably happily well inside a car so this may not be a problem.
Here is one example complete with software:
http://custom1.farnell.com/cpc/searc...eyword=IT44475
Manufacturer's site:
http://www.haicom.com.tw/