I'm currently driving my dad's 2006 VW Jetta TDI (he's driving my 1998 VW Jetta TDI at the moment, long story on that). The new Jettas/Golfs/etc. have a dash diffuser vent that's mounted above the central dash vents. I have my detector mounted just below the line of sight for the rear view mirror and the air flow from the diffuser blows right on it, keeping it pretty cool when operating.
The summer sun here can be vicious (for example, last summer I left a cassette tape on my car seat and the sun melted it into an unusable mess) and I've had detectors stop working for no other reason than they cooked under the sun so anything that will keep my detector cool and working is gladly welcome. 8)
Of course I remove it when parked and the sunshade is up. I'm planning on buying this car in the near future when my dad decides to upgrade.