My TomTom XL Live works fine under the dash and in town or slowing for junctions I can hear Samantha ordering me about!
I tried the Tom Tom adhesive mount plate under the dash, which placed the unit pretty well for me, but vibrated and so I took some earlier advice and went down the RAM mount route with it clamped to the under dash crossmember. A bit of a fiddle fitting it but it works very well and does not vibrate.
