They're a little pricey but Jawbone's Big Jambox is incredible. 
It sits on the dash and I play music from the iPhone. 
It doubles as a speakerphone that you can actually use in a vanagon. 
The nice part is you move it to the top of the stove or out to the picnic table when you're parked.

Shawn
87 Syncro "Eeyore"
78 Champagne Edition Bus
80 Aircooled Westy L "Tigger"