Sorry for resurrecting such an old thread but I have just bought a Tomtom 920T and I was searching for mounting solutions for my Mk4 Golf..
I wanted a fixed power mount from which the Tomtom could easily be removed, in my line of sight, above dashboard level, hardwired to the vehicle ignition feed, with no trailing wires and so far as possible all the wiring out of sight. I do not like mounts that cover air vents or switches or suction mounts and as my phone already uses a "Brodit" mount to the side of the radio, that was not an option and it would not have been in my eye line anyway.
The Brodit Power mount for the 920T looked good but I needed something substantial to mount it to, not the suction mount that came with the Tomtom and not something that was going to cover a vent or switch and most definitely not something that was going to mark or damage my car !
The bottom of the windscreen pillar on the drivers side was where I would like to have the unit situated. My solution to mounting it there was to fabricate my own alloy bracket and attach it to the substantial under dash mounting plate which can be accessed via the fuse box cover on the drivers side. The new bracket I made runs up between the fuse box cover and the end of the dashboard just to the right of the steering wheel (just where I wanted the Tomtom to sit ! ) and it even allows the top of dash vent to blow behind it & over onto the side window !
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To this bracket I then mounted a Brodit Power Mount (which has a swivel to allow some movement) and I ran the power wire back down under the new bracket into the fusebox area where I picked up a switched feed and earth. which I soldered onto the cigar plug contacts, insulated and tucked it up out of the way with a cable tie. (NOTE the Tomtom 920 does not use a 12v supply & it requires the cigar adapter to reduce the voltage to the correct level).
The TMC aerial was hidden away (despite indications that it would not work if doing this) by running it up the inside of the window pillar and back towards the back of the car for it's full length and "the egg" part sits just behind the new bracket. It seems to work fine picking up traffic information within a couple of minutes of switch on and so far on the few trips I've done out of main city areas it seems to continue to receive a signal & update without any problems. 
The mount itself is very rigid (but needs a proper coat of satin black paint rather than the quick coat I gave it) and does not damage my car in any way. The Brodit Powered Holder allows a degree of movement with it's swivel head while the Tomtom 920T easily clips in and out of the mount without any great difficulty. The Tomtom unit is set to power down whilst giving a ten second countdown on the screen, allowing me to cancel shutdown if I want, whenever the ignition is switched off. :) It is also set to carry on from whereever it left off on the next power up which allows me to stop en-route, power down the Tomtom and not have to worry about setting up the route again when I'm ready to continue. 
I'm not too bothered about the holder (and my Talex Unit for that matter) being in sight of others as I always remove the Satnav & Talex unless I'm parking in my own garage at night or at the secure indoor parking at my office.
Not perhaps what everyone would go for but it's just exactly what I wanted and I thought I'd share this with anyone else looking for a similar kind of mounting solution in a Mk4. 


