The two alternatives for aftermarket mentioned by Monty have both been in use by me.
Used a Becker 'radio'-type navsys - Becker Traffic Pro (no longer for sale, but very much like this one
(sorry - link in Dutch but at least it gives an impression).
Advantages: TMC, very precise navigation because of coupling to speedo, built-in gyros and excellent extermal antenna. Complete for navigation in Europe; once route has been calculated then out goes the CD and it's ready to play tunes (didn't use that much though - had a cd changer for the music and the slot in the radia was dedicated to the navsys).
Drawbacks: expensive to buy; cost to build in & out when you change cars. No maps, just pictograms & voice to tell you to take a left etc - so basically, on new territory, you have no clue as to where you are relative to anything else you might assume to be somewhere near...
Now, in the 156, I use the Traffic Assist
, also from Becker. Like it a lot!
Fast navigation, nice maps, good selection of POI, easy to use, the whole of Europe (27 countries) on a 2Gb SD card.
Drawbacks: no TMC (it's an extra but I don't like the new Becker TMC software (may have improved in the meantime)) & sometimes slow to pick up satelites.
I don't use it a huge lot - but anywhere I go for the first time, I do & it hasn't failed me once in 15.000 km I've approx. done with it (Spain, France, Belgium, Germany & Netherlands of course).
I also use it rather consistently whenever I have appointments - its ETA is fairly precise (a bit optimistic perhaps) so I have a chance of informing the people I'm meeting that I'll be late (not telling them, of course, that this is due to the detour I'm taking because the 156 tells me to
The previous one (the built-in) was in use all of the time.
The traffic assist is mounted on the windscreen. No visible marks so far, but I'm keeping my windscreen spic & span anyway...
I have considered other mounting places and www.Brodi.se
has quite an extensive set of clamps and stuuf for mounting these gadgets in other places, but since I'm content with the screen mount I've left it at that.