I have a laguna 2, 1.9dci dynamique.
I use an IPAQ 4700, running tomtom 5, with a bluetooth GPS. In the boot I have fitted a 3 outlet power assembly in the l/h side of the boot where the elastic thingy and small shelf is. The GPS mouse sits next to it with a powered GPS reradiating antenna fitted inside the hatch window (used to be outside on the roof, but it attracted vandals) I have a seidio powered & amplified mount fitted to the windscreen.
All in all it works well, and did a good job in France recently.
The goona has an athermic heat resistant screen, it blocks signals for sure but there is still enough bouncing around in the car for an internal sat nav to work. With the GPS mouse sitting on the dash I get 4 - 5 medium strength sats, on the rear shelf i get 8 medium sats, with my re-radiating set up I get 9 full strength.