Tried the following...
1. Top of the dash in the corner (i.e. in front of the tweeter) - crap sound at both ends, bad idea.
2. Top of the A Pillar next to the side window - better but not great - a bit of wind noise.
3. Top of the A pillar, tucked away next to the windscreen (i.e. around the corner from position 2) - best so far and very acceptable.
The Fiscon comes with two little mounting clips of different sizes and also a small sticky backed mounting ball. The smaller of the clips is pretty discreet and good for the pillars, but if you put it between the clocks the sticky backed ball will be perfect.