decent idea, awful execution. i got some tips from Dennis at SPD, which i've copied below.
There is a diagram in the box (or should be), to hold against the lower dash and see where you have to cut a semi circular piece out, there are obviously some connections inside underneath for the arm to plug into. The dash material is pretty tough, I cut out a hole for a switch, with a model knife - it broke two sharp #11 blades.
So you get that far, and plug it into the lower dash, and the backward end is sort of alongside the gearshaft cover.
You have to get a piece of sticky paper and stick it on the side of the gearshaft cover. Draw around the arm end (a circle).
Remove the gear knob - just pulls upwards (do NOT hold the side button if you have that).
Remove the gearshaft cover (first remove a screw hidden under the handbrake lever).
Cut out the hole (dont make it bigger than necessary).
Trial fit that the arm will fit inside your hole.
Replace cover, fit the telephone arm, refit the gearknob.
I haven't done this, Smart Amsterdam (Holland) described how they do it.
When you have finished you have to find a holder for your particular telephone, and screw to the arm.