Hi Tomrod -
One option I've tried is plugging a powered, external speaker into the laptop headphones jack.
I use this $15 RadioShack speaker
http://www.radioshack.com/product/index.jsp?productId=2062620
You also need an MP3 cable like the one shown here
http://www.radioshack.com/product/index.jsp?productId=2102949
A drawback to the speaker is that it uses a 9volt battery - but I think you'll find it lasts for a very long time with normal usage. This particular speaker comes with a volume control, which is a nice feature.
Of course, if your car is a newer model and has an MP3 audio jack built in to the stereo, you can skip the speaker and plug directly from the headphone jack to the stereo. That way in 'AUX' mode, you're playing S&T directions right over your car stereo.
Drawback here is you can't listen to music/talk on the stereo at the same time....but if you need the turn by turn, maybe a good idea to skip the music anyway.
If you do plug into the dash, and absolutely need some music - you can buy an MP3 'Y' cable - then - plug your laptop and MP3 player into the 'Y' and funnel both into the stereo (you'll want short cables for laptop/MP3 player end).
Hope that helps!