Why not mount it in the cubby hole infront of the gear shift?

I did this and if you place something in front of it then no one knows youve got a Tomtom sitting there, otherwise if you really want you can just unmount it and hide when you leave the car. Reception is unaffected from that position too (i installed the external antenna and discovered the performance was better without it!)

Much better option as the suction cup marks give away the fact youve got a Satnav lurking somewhere in the car.